Slowing the Flow in the Rivers Ouse & Foss
Water and Environment Management Framework Lot 3 – Engineering and Related Services
Slowing the Flow in the Rivers Ouse & Foss
July 2017 Document overview Capita AECOM was commissioned by the Environment Agency in May 2016 to undertake the York Flood Management which has comprised this review of the potential for upstream storage and Natural Flood Management to provide opportunities to manage the flood risk faced by the City of York.
Document history Version Status Issue date Prepared by Reviewed by Approved by 0 Draft March 2017 A Gee G Knott C Lomax 1 Proof Copy April 2017 A Gee G Knott C Lomax 2 Final July 2017 A Gee G Knott C Lomax
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ii Contents 1. Introduction 1 2. The Upstream Catchments & Flood Risk in York Today 6 3. The 5-Year Plan 13 4. Catchment Scale Measures to Reduce Flood Risk in York 18 5. Upstream Storage 22 6. Natural Flood Management 33 7. Engineering Measures 44 8. Development and rural land use planning 48 9. Flood Forecasting & Warning 51 10. Total Catchment Management 56 11. Summary 59 Figures Figure 1.1: Response to the Boxing Day 2015 flood event 1 Figure 1.2: The catchments upstream of York 3 Figure 1.3: Flooding on the 28th December 2015 looking upstream from the confluence of the Rivers Ouse and Foss 4 Figure 1.4: Flooding on the 28th December 2015 on the River Foss looking west 5 Figure 2.1: The steady increase in the annual maximum levels at the ‘Viking’ river level recorder in central York 10 Figure 3.1: Existing flood defences 15 Figure 4.1: Location of river gauging stations reviewed 19 Figure 4.2: Tributary hydrographs resulting in the Boxing Day 2015 flood event 20 Figure 5.1: Location of existing reservoirs upstream of York and existing washlands 25 Figure 5.2: Myton Ings (River Swale) during November 2000 event, looking upstream 27 Figure 5.3: Structures restricting the Foss flow downstream 29 Figure 5.4: River Ouse, November 2000 looking upstream of York 32 Figure 6.1: The impact of land management on flood risk. NFM seeks to reinstate / emulate the catchment on the right 33 Figure 10.1: Flood risk management activities 56 Figure 10.2: Total Catchment Management 58 Figure A3: Catchment scale classification of NFM measures, coloured arrow indicates increasing understanding and reliability (adapted from Thorne et al., 2007) 65 Figure A4: Catchments feeding the River Ouse through York 72 Figure B1.1: River Foss NFM Study area in relation to surrounding catchments and the City of York 92 Figure B1.2: Overview of Foss NFM Study methodology 93 Figure B2.1: River Foss, tributaries and key catchment features 94 Figure B2.2: LiDAR coverage in the Foss catchment 95
iii Figure B3.1. Reach Identification and Numbering 99 Figure B3.2. Numbering of sub-reaches within the Foss catchment. 100 Figure B3.3. Modelled Extents. 103 Figure B3.4. Flood Modeller routing model of the Foss Catchment. 106 Figure B6.1. Location at which modelled results were taken. 116 Figure B6.2. Impact of Scenario 5 interventions within the River Foss sub- catchment for the 1% AEP event 117 Figure B6.3. Impact of Scenario 5 interventions within the Tang Hall Beck sub-catchment for the 1% AEP event 118 Figure B6.4. Impact of Scenario 5 interventions within the Osbaldwick Beck sub-catchment for the 1% AEP event 119
Tables Table 4.1: River gauge data 19 Table 5.1: Upstream storage volumes required for level reductions at York 22 Table 5.2: River Foss Storage Volumes 23 Table 5.3: Existing Upstream Storage Reservoirs 23 Table 5.4: Existing washlands 27 Table A5: River and catchment based NFM measures (adapted from Natural Flood Management Handbook, SEPA, 2015) 64 Table A6: Barriers to implementation of NFM measures 71 Table A7: Comparison of the SUNO, Foss and Pickering catchments during the Boxing Day floods (December 2015). 74 Table B2.1: Water Framework Directive Status of waterbodies within the Foss catchment 97 Table B3.1: Reach typology classification (after Chow, 1959) 101 Table B4.1: List of NFM measures for identification within the catchment 108 Table B4.2: Opportunities and constraints for Woody Debris 109 Table B5.1. Adjustments to Manning’s ‘n’ to represent NFM measures within the Foss catchment (after Arcement & Schneider, 1984) 112 Table B6.1. Impact of Scenario 5 interventions at downstream end of modelled watercourses. 119 Table B6.2. Impact of intervention of peak flow at downstream end of modelled watercourses when compared to baseline, broken down by scenario. 120 Table B6.3. Impact of intervention on timing of peak flow at downstream end of modelled watercourses when compared to baseline, broken down by scenario. 121
Appendices Appendix ANatural Flood Management Literature Review Appendix BNatural Flood Management Technical Report
iv 1. Introduction
Storms Eva and Frank in December 2015 brought record rainfall across Yorkshire with over 600 households and businesses flooding in York on the 26th December 2015. The flood event was amongst the most significant in Yorkshire since current records began: many locations recorded their highest ever flood levels. The river level on the River Ouse in York reached its second highest peak in the recent record, whilst the River Foss recorded its highest ever river flow.
Since December 2015, the Environment Agency, City of York Council and partners have taken action to reduce the risk of flooding in York. This has included:
x Immediate recovery: the immediate response during the event, and in the aftermath of the flooding, including increasing the pumping capacity of the Foss Barrier; x Short term: preparing a 5-year Plan to manage flooding in York in the short term through flood alleviation measures and providing consistent standards of protection within the City of York area; and x Long term (this document): a review of means by which to manage the risk of flooding in York over the next 100 years, through flood alleviation measures upstream of York. These are shown graphically in Figure 1.1.
Figure 1.1: Response to the Boxing Day 2015 flood event
Chapter 2 of this document describes the catchments of the Rivers Ouse and Foss which flow through York. It discusses the flood risk these rivers pose, documents the recent historical flood history and introduces how river flows are changing over time. The chapter also identifies other sources of flooding in York.
Chapter 3 presents a high level summary of the existing flood defence assets in York and the Environment Agency’s 5-year Plan to manage flooding in York. The 5-year Plan comprises possible interventions through the City of York to provide additional measures to alleviate the effects of flooding. This document has been designed to supplement the 5-year Plan, to consider holistic solutions upstream of the city to alleviate flooding in York for the next 100 years.
1 Chapter 4 introduces the catchment scale measures that have been considered as part of this document to slow the flow that reaches York during a flood event, which are covered in the subsequent chapters and discusses the hydrology of the upstream catchment identifying characteristics requiring further consideration if catchment-scale measure are to be implemented upstream of York:
x Chapter 5 looks at providing washlands to store water upstream of York during a flood event; x Chapter 6 reviews the potential of Natural Flood Management (and sustainable drainage in urban areas) to provide benefits to York; x Chapter 7 provides a high level consideration water level control and other engineering options that could be implemented outside of the city but would benefit the city; x Chapter 8 touches on the role of development and rural land use planning; and x Chapter 9 discusses the role of flood forecasting and warning. Chapter 10 presents a vision for a total catchment based approach for the River Ouse (and it’s tributaries) and the River Foss. As described in the River Basin Management Plan (RBMP)1 for the Humber river basin district a catchment based approach helps to bridge the gap between strategic management planning at river basin district and activity at the local water body scale. It is central to the Environment Agency’s Flood and Coastal Erosion Risk Management Strategy for England and this document provides a strategic look at measures that can be implemented in the Ouse and Foss catchments upstream of York, to help manage the flood risk faced by the City. Figure 1.2 illustrates the river catchments upstream of York: these correlate to the area covered by the Swale, Ure, Nidd and upper Ouse catchment partnership identified in the RBMP which is hosted by the Yorkshire Dales Rivers Trust.
The outcomes of the plan are a series of recommendations for the Environment Agency and it’s partners to consider by which to manage the risk of flooding in York over the next 100 years through implementation of a total catchment management approach.
The Environmental Assessment of Plans and Programmes Regulations 2004 (SEA) regulations do not apply to this plan, as it is not a plan ‘required by legislative, regulatory or administrative provision’ and therefore does not fall under the definition of ‘plans and programmes’. However, we will ensure an appropriate level of environmental assessment will be undertaken for any actions where there is potential to damage the environment. A stage 1 Habitats Regulations Assessment (HRA) has been undertaken, concluding that this study does not result in any actions on the ground that could result in effects on the European sites. Some actions will require further investigation and assessment under specific legislation and this will be determined on a project by project basis.
1 Water for life and livelihoods – Part 1: Humber river basin district, River basin management plan; Defra and EA; December 2015
2 Figure 1.2: The catchments upstream of York
3 1.1 The Boxing Day 2015 Flood Event
The flooding of Boxing Day 2015 was the trigger for this document and the following section provides further background information regarding the causes of the flooding. The flooding arose from very high levels of rainfall recorded during November and December 2015 which had the effect of saturating the catchments upstream of York. Storm Desmond at the start of December was followed by Storm Eva which in turn was followed by the average monthly rainfall for December falling within a 48 hour period.
As a consequence, the level on the River Ouse reached its second highest peak on record at Skelton and the peak flow of 544m3/s has been assigned an Annual Exceedance Probability (AEP) of 2-2.5%. Through the centre of York the level at Viking on the River Ouse rose to a peak level of 5.19 metres, just below the 5.4 metres recorded in November 2000.
The rainfall and high flows in the Foss catchment which were exceptional with an intense cell of rainfall occurring at the top of the catchment. The peak flow at Huntington on the River Foss was 40m3/s, the highest on record which has been assigned an AEP of less than 0.5%.
Figures 1.3 and 1.4 illustrate examples of the high river levels and flooding experienced during December 2015.
Ouse Flow Foss Flow Direction Direction
Foss Barrier
Figure 1.3: Flooding on the 28th December 2015 looking upstream from the confluence of the Rivers Ouse and Foss Source: Peter Smith Photography
4 Ouse Flow Direction
Foss Bank Car Park
Foss Flow Direction
Foss Islands Road
Figure 1.4: Flooding on the 28th December 2015 on the River Foss looking west Source: Peter Smith Photography
5 2. The Upstream Catchments & Flood Risk in York Today
Flooding can come from many sources. It can occur when rainfall overwhelms local drainage and water flows overland, it can come back out of sewers and drains, and in some places it can come from groundwater or a high natural water table. However, the greatest flood risk to homes and businesses in York is from the rivers Ouse and Foss.
The River Ouse flows from the north-west of York, with the River Ure taking the name Ouse shortly after it has been joined by the River Swale and just before it is joined by the River Nidd. The Swale, Ure and Nidd and their smaller tributaries drain an area of around 3,300km2 (to Skelton gauging station), including large parts of the Yorkshire Dales, the Vale of Mowbray and the Vale of York, and a small part of the North York Moors. Together they provide the majority of the water that flows through York.
The River Foss is a much smaller river with a catchment area of less than 200km2. The River Foss rises north of York adjacent to Oulston Reservoir. It then flows in a roughly southerly direction through agricultural land before running through the east side of York and joining the Ouse close to Clifford’s Tower.
The eastern boundary of the City of York Council area is the River Derwent and there is flood risk here, particularly to the village of Elvington, but it is beyond the scope of this document to consider the Derwent and Rye catchments. Other plans and measures will help to reduce flood risk to communities on this side of the city.
2.1 River Ouse
The Swale, Ure, Nidd and Ouse (known collectively as ‘SUNO’) catchment upstream of York is approximately 3,300km2 and is mainly rural. Its northern and western extent in the North Pennine Moors and Yorkshire Dales is characterised by steep uplands and sharp river gradients. The eastern and southern region of the catchment is relatively flat and characterised by the undulating plains of the Vale of York. A tidal influence is only seen downstream of Naburn Weir, just at the southern edge of the City of York Council area.
The catchment has a wealth of environmental designations of international, national, regional and local nature conservation importance, several hundred Scheduled Ancient Monuments and thousands of Listed Buildings. The waterlogged conditions of the burial environment in low-lying parts of York are a key part of the preservation of exceptional archaeological remains in the city.
Agricultural land in the catchment is mainly of good to moderate quality, with the majority being used for arable agriculture or improved grassland. Urban land use occupies less than 1% of the catchment. Large urban areas include Richmond, Thirsk, Northallerton, Ripon, Harrogate and York, and there are numerous smaller rural communities.
In the past the Ouse and lower Ure have provided a commercial trade route into the towns and cities of North Yorkshire. Smaller watercourses have been less used for navigation, but provided an important source of water for homes and farming and of power for early industry. This led inevitably to development on the floodplains, and there is a long history of flooding in
6 many of our communities. Flood defences and flood alleviation measures have been developed over time, although most larger schemes have only come into being in the latter half of the 20th-century and into the 21st-century.
Flood defences in the SUNO catchment today come in various forms, and have been influenced over time by social and economic factors as well as by the ever-developing science of flood risk. Floodplain controls and storage areas vary from low rural embankments (many of which can be seen on the Swale) to formal reservoirs (such as Gouthwaite on the Nidd) and washlands (such as Aldborough Ings near Boroughbridge, or Clifton Ings in York). Walls and embankments have sometimes been built to defend very specific areas of homes and businesses (such as the walls on North Street in York) and have sometimes been built as part of a larger scheme combining many different elements (such as the Ripon flood alleviation scheme, which combined walls, earth banks, increased flow under bridges and an upstream storage area).
2.2 River Foss
In national terms the River Foss is often treated as part of the SUNO catchment, but for York it represents a different source of risk and has a very different character to the Ouse. The Foss has only a very small ‘upland’ component in the Howardian Hills, and the majority of the catchment is characterised by agricultural land of good or moderate quality. The Foss enters York at Strensall, and from here to the confluence with the Ouse it takes on a much more urban character.
The largest tributaries to the Foss within York are the Tang Hall and Osbaldwick Becks, and in medieval times these merged with the Foss to form the King’s Pool on the east side of the city centre. Over time and particularly in the late 18th-century this pool was in-filled and the river was straightened into the current channel and, with levels maintained by a lock and sluice at Castle Mills, the Foss was used for commercial navigation until the late 20th-century.
Having a relatively small catchment area in a drier part of the country, the Foss has never been subject to high flows as often as the Ouse. Oulston Reservoir at the very top of the Foss was built in the late 18th-century as a means to provide extra water into the river and maintain levels for navigation in dry periods. Nevertheless, there have been serious floods on the Foss. In 1947 flooding to homes and businesses was attributed in part to water being unable to flow out of the Tang Hall Beck system, and improvements including large new culverts were built in the 1950s.
Other floods in the Foss catchment have been more directly related to floods on the Ouse; in the past when water levels in the Ouse rose higher than Castle Mills lock the greater volume of water in the Ouse would force flows back up the Foss and flood properties in the Foss Islands and Huntington Road areas. After a flood of this nature in 1982 the Foss Barrier was conceived and built, keeping Ouse water out whilst Foss flows were pumped away at a managed rate.
In 2015 exceptional flows down the Foss overwhelmed the capacity of the pumps, and flooding was again seen around the Foss. An upgrade of the pumps in 2016-17 will significantly reduce the likelihood of this reoccurring.
7 2.3 Fluvial Flood Risk
York has a long history of flooding, with written records of floods stretching back as far as the 13th-century. Before Naburn Lock was built there was some tidal effect seen in the city, but this was relatively small and the predominant flood risk has always come from high flows coming down the River Ouse. The most likely cause of these high flows has traditionally been seen as snow-melt or winter storms across the Yorkshire Dales, but some of the highest flows in recent years have been in the summer or early autumn.
There is a clear east-west variance in the SUNO catchment in terms of the amount of rainfall received. Rainfall averages 2,000mm per year in the headwaters of the River Ure whilst, in contrast, areas to the east of Ripon receive approximately 600mm per year. As the west receives a much larger volume of rainfall it follows that larger river flows are most often generated in the western upland catchments; it is often the case that there can be flooding in York when almost no rain has fallen locally. However, this is not an absolute rule and serious floods can be generated by rain falling in the lower catchments through the Vale of York. These lowland-sourced floods will arrive more quickly in York, with less time to forecast and prepare for their impact.
The nature of the flooding experienced in York depends on how rain has fallen across the three catchments of the Swale, Ure and Nidd. In one instance the three flows will arrive across a period of time, leading to a flood of long duration but lower peak levels in York. In another instance the three flows will coincide in timing, leading to a flood of shorter duration but greater peak levels. The second scenario is generally considered to be the most damaging, although floods of long duration can be more disruptive to businesses and the wider economy.
2.3.1 Recent Fluvial Floods The floods of March 1947 are now on the edge of living memory, but formed a reference point for flooding on the Ouse for some time after. They were caused primarily by the melting of a large volume of snow that had fallen across a prolonged cold spell in January and February that year. The army played a key role in the immediate emergency response, but the country was not in a position to start building big flood defences in the post-war era. National priorities at the time were focussed on food and industry, and tellingly the official government report into the floods was called ‘Harvest Home’.
Through the 1950s-70s there were smaller floods through York, and flood risk management powers passed from the Local Authority through to River Boards, River Authorities, and then to Regional River Authorities. Some improvements were made in this time to embankments on the lower Swale and the lower Ouse, but it was following the floods of 1978 that a series of flood defence improvements in York were initiated by the Yorkshire Water Authority. This programme of work forms the basis of the current flood defence arrangements in the city.
Serious flooding in 1982 added impetus and new data to support the programme of flood defence building, and the problem of water from the Ouse flooding up the Foss was a key concern. This was addressed by the construction of the Foss Barrier in 1987.
The flood in autumn 2000 reached the highest level ever recorded on the River Ouse and there was widespread flooding of the river’s major tributaries. This flood followed a period of unprecedented rainfall; the autumn of 2000 was the wettest since rainfall records began in 1766. Flood defences protected many areas, but there was flooding of 540 properties in York.
8 In September 2012 the River Ouse again rose to a level of over 5 metres above normal, and the city’s flood defences successfully protected more than 1,000 properties. There were however over 200 properties directly affected by flood water including residential properties at Leeman Road, Lower Ebor Street, Alma Terrace, Fulford and Naburn.
Heavy rainfall through December 2015 again led to flooding in a number of communities, but the most severe and widespread flooding was seen between Boxing Day and New Year in York. Levels on the Ouse again rose above 5 metres, but there was also very heavy local rainfall over the Foss catchment. The River Foss at Huntington recorded the highest ever level at around 3.57m, topping the previous recorded level of 2.91m by over half a metre.
Working at full capacity, the Foss Barrier was unable to cope with the level of flow in the River Foss and was at risk of being overwhelmed and rendered inoperable. As a result the decision was made to raise the barrier gate. This slowed the rate of rise on the Foss and reduced the maximum level of the flooding, providing more time for emergency services to respond and reducing the overall damage done by the flooding.
2.3.2 Fluvial Flood Modelling The Environment Agency’s knowledge of any river catchment is based on a combination of historic evidence and computer modelling. Control of the hydrological inputs to computer models allows a common standard to be applied across different catchments, and a flood model can be used to simulate the effects of floods greater than any that have ever been recorded. Climate change is considered in three epochs, known as 2020s, 2050s and 2080s. For York this means that we have modelled increased inputs of 20%, 30% and 50% respectively. This indicates how we might have to prepare developments and flood defences for the future.
Flood modelling technology has developed rapidly as computing power has increased in recent years, and the Environment Agency regularly reviews and updates flood models as appropriate. The Ouse and Foss were modelled separately in 2003-4 and improved in 2009, and in 2014 a review of the existing data concluded that a comprehensive update was required. This included a detailed survey of defences and channels in the city in 2014-15 and the creation of a new single model of the Ouse and Foss, known as the York Detailed Model. This model was finished in draft version in December 2015 and some of the draft outputs were used to inform the emergency response to the flooding on Boxing Day 2015. The model would have been finalised in early 2016 if the winter flooding had not occurred.
The flood on the Foss in late December 2015 was significantly larger than anything previously on record, and it would not have been credible to publish new flood maps without accounting for this new evidence. After the Environment Agency had collated all the data from the flooding, consultants Mott MacDonald validated and re-calibrated the York Detailed Model. The new model was completed in October 2016 and is currently the best available representation of flood risk from the Ouse and Foss in the city.
Not all smaller watercourses are included in the York Detailed Model, and a separate study has also recently been carried out on Holgate Beck by City of York Council.
2.3.3 The River Ouse is Changing Over the past 100 years the River Ouse has seen a steady upward trend in the annual maximum levels. The reasons for this are complicated, and there is no single clear reason for the trend. Increased water runoff from urban developments and changes in agricultural drainage practices are both likely to be factors, as is climate change. Although this trend has
9 been apparent for some time it has become particularly clear in the last 30 years. As new evidence is collected we have to reassess the adequacy of our flood defences. Walls and embankments built to what was believed to be a high standard in the 1980s are now thought to be lower than the desired height.
Figure 2.1: The steady increase in the annual maximum levels at the ‘Viking’ river level recorder in central York
Climate change is recognised by the UK government as being likely to cause heavier rainfall events and increased river flooding and flash flooding. It will also lead to sea-level rises, and a coincidence of high tides and high river levels could exacerbate flood risk on the lower Ouse and in the Naburn and Acaster Malbis area of York.
The latest flood modelling of York accounts for three potential climate change scenarios, showing a worsening situation over the next 100 years. Flood defences can have a lifespan of up to 50 years before they need replacing, and so we aim to build all new flood defences with an allowance for climate change built in. If, for example, we are building a flood defence to protect against a flood with a 1% chance of occurring this year then it also needs to protect against a flood with a 1% chance of occurring in 50 years’ time. In York the future 1% probability flood could be up to 60cm higher than the current 1% probability flood, and so this can represent a significant addition to a flood defence wall or bank.
Climate change predictions are not certain, and it is not always possible or desirable to build flood defences higher than their current level at the moment. One way to address this is to allow for easy future improvements. The new flood defences at Water End have been built in such a way that the foundations and walls are strong enough that they could be easily raised slightly higher in the future without extensive rebuilding. In other cases this will not be possible, and climate change will have to be addressed by other measures. This is a likely role of Natural Flood Management and flood storage areas in the SUNO catchment, and the opportunities for this are addressed in more detail in sections five and six.
10 2.3.4 Flood Risk and the Wider Catchment This report is focussed on reducing flood risk in York, but the city cannot be treated in isolation when discussing flood risk and the benefits of flood alleviation measures. The River Ouse carries water collected across a wide expanse of North Yorkshire, and measures that reduce flooding in upstream communities could also benefit York. For example, recently completed flood defence schemes in Ripon and Catterick include elements of flood storage to ‘slow the flow’. The affect that these measures have in York is so small as to be un-measurable in current flood modelling, but as such schemes, along with Natural Flood Management, become more common they will have a cumulative impact.
Downstream of York the next large centre of population, Selby, has quite a different character of flood risk due to the strong tidal influence on the River Ouse there. There are also extensive controlled washlands and floodplains between York and Selby, and these change the nature of flooding in the area. Nevertheless, changes to flood risk management in York could have an influence on events downstream and we must always be aware of this when making any plans for the future. A key principle of modern flood risk management is that we cannot simply transfer risk – we can’t improve the situation for one population by making it worse for another – and this must always been born in mind when discussing York.
2.4 Flooding from local sources
Management of ordinary watercourses (smaller streams and becks that are not designated Main River) falls under the remit of the Lead Local Flood Authority or, where present, an Internal Drainage Board. Internal Drainage Boards (IDBs) exist in agricultural areas, and have permissive powers to manage water levels within their drainage districts. Around York there are a number of IDBs or groups of IDBs managing land around all the main watercourses.
Lead Local Flood Authorities have a wide-ranging role, with permissive powers to manage flood risk from surface water, ordinary watercourses outside of internal drainage districts, and groundwater. Lead Local Flood Authorities also manage the drainage on the majority of local highways.
In North Yorkshire the Lead Local Flood Authority is North Yorkshire County Council, and in York it is City of York Council. Both authorities have published a Local Flood Management Strategy that covers each of the risks they manage in more detail. City of York has also published a specific Surface Water Management Plan.
The Surface Water Management Plan for York acknowledges that surface water flooding and local drainage issues are generally less well understood in York than fluvial flooding, but also notes that there is very little record of surface water having caused significant flood problems historically.
Flooding from local sources can combine with fluvial flooding to exacerbate a situation, and this is something that must be born in mind when river-side flood defences are being developed. The removal of fluvial flood risk can sometimes reveal another problem that had been masked by the great quantity of river water in the area.
The City of York Council Strategic Flood Risk Assessment assesses the different levels of flood risk in the local authority area and maps these to assist with statutory land use planning. It
11 provides concise information on flood risk issues and assists planning officers in the preparation of the Local Plans and in the assessment of future planning applications.
2.5 Flooding from reservoirs
There are 33 reservoirs in the SUNO catchment, used for water supply, storage and recreational purposes. The Flood Risk Maps for Reservoirs show that approximately 6000 people are at risk from flooding resulting from a failure of a reservoir in the catchment, and the greatest areas of risk are upstream of York. Although there is some risk to riverside properties in York, the distance between the reservoirs and the city means that a large amount of water will have dissipated across the floodplain before reaching the city.
It should be noted that although the consequences of reservoir flooding are high, the probability of reservoir failure is very low.
12 3. The 5-Year Plan
Recent flood events, including the Boxing Day 2015, event and the increase in frequency of high river levels on the River Ouse have demonstrated that the flood defences in York are no longer providing the standard of protection that they were designed for. Consequently, the Environment Agency and its partners have aspirations to provide a suite of improved flood defences through the City of York by 2021. The 5-year Plan is designed to deliver these defences using the additional £45 million booster funding that the government has allocated to the Environment Agency to better protect 2,000 properties across York.
Hard defences (such as floodwalls) cannot be built and raised higher indefinitely, however the 5-year Plan and government funding presents an opportunity to improve the existing defences and provide a consistent standard of protection across York. Climate change presents additional challenges to be considered in both the 5-year Plan and the longer term. This document dovetails with the 5-year Plan to consider additional opportunities to supplement hard defences that can manage the risk of flooding in York.
The 5-year Plan was published by the Environment Agency in November 2016 setting out a range of possibilities that could be considered to achieve a better standard of protection from flooding across York. Detailed assessments are to be undertaken during 2017 and 2018 to understand the feasibility, cost and appropriateness of these interventions.
The following chapter provides a high-level summary of the existing flood defence infrastructure that currently exists through York and the possible interventions that could be delivered through the 5-year Plan.
3.1 Existing Assets
York has a number of existing flood defence assets protecting people and property from flooding. Figure 3.1 illustrates the locations of the existing flood defences as listed below from upstream to downstream:
x Clifton Ings - a formal washland capable of storing water to reduce water levels downstream in the centre of York; x Lower Bootham Flood Alleviation Scheme - comprises a series of flood embankments and floodwalls from north of Burdyke Beck to Museum Gardens, a pumping station prevents the Burdyke Beck backing up and flooding when free discharge into the Ouse is not possible; x Water End Flood Alleviation Scheme - a floodwall runs along the western side of Water End from the junction with Landing Lane to opposite properties on Forth Street, and a flood embankment runs south from Water End, to St. Barnabas Church of England Primary School. Holgate Beck Pumping Station prevents the River Ouse backing-up Holgate Beck; x North Street Flood Alleviation Scheme - provides a line of defence from Lendal Bridge to Ouse Bridge. At Lendal Bridge a flood gate forms a seal across an opening in the bridge from which a floodwall extends south tying into property walls further downstream; x Foss Barrier – located at the downstream extent of the River Foss, just upstream of the confluence with the River Ouse, prevents the Ouse backing up the Foss and causing
13 flooding. A pumping station then pumps flow from the River Foss to the Ouse. In response to the Boxing Day 2015 flooding, the pump capacity is in the process of being upgraded to 50m3/s – the capacity is currently 40m3/s. In conjunction with the barrier and pumps, there is a floodwall around St George’s Field Carpark preventing the River Ouse bypassing the Barrier; x Lower Ebor Street – a floodwall protects Lower Ebor Street from flooding; and x Middlethorpe Ings - is a modified floodplain designed to store flood water and lower water levels in York. Other infrastructure and assets such as: culverts, trash screens, penstocks, flap valves, “de facto” defences, etc. exist throughout York. These are not covered here because the discussion has focused on the larger, more obvious flood defences that are operated and maintained by the Environment Agency.
14 Figure 3.1: Existing flood defences
15 3.2 5-year Plan
As part of the 5-Year Plan, ten ‘communities’ have been defined within York’s administrative boundary where there is a risk of flooding. Below is a list of possibilities that are being considered for each community to achieve a better standard of protection.
3.2.1 Foss community x The Foss Barrier and pumping station are currently being upgraded to significantly reduce the risk of flooding in the Foss Basin; x Opportunities to store water upstream of the city are to be explored to slow the flow of the Foss and reduce the need to construct additional defences within the Foss Basin; and x In key locations work may be undertaken in the river channel to improve the flow of water, particularly through the city centre.
3.2.2 Poppleton community x A new floodwall between Main Street and Ferrymans Walk, including flood gates for access and a pumping station.
3.2.3 Clifton and Rawcliffe community x Construct a new pumping station where Blue Beck enters Clifton Ings and increase the capacity of the existing Burdyke Beck pumping station; x Raise the existing embankments at Clifton Ings, Lower Bootham and St Olave’s School; x Build a floodwall or embankment to the rear of Government House Road; and x Raise the height of the Almery Terrace floodwall.
3.2.4 Holgate community x Build an embankment along Gale Lane Drain; and x Raise the height of the Holgate Beck pumping station and access road to improve its resilience to flooding.
3.2.5 York city centre community x Increase the height of the footpath between Scarborough Bridge footbridge and Earlsborough Terrace and the roundabout on Tower Street; x Build new flood gates and walls in the post office car park and Memorial Gardens; build demountable walls along Skeldergate, Queen Staithes and Kings Staithes; install new defences from Lower Friargate to South Esplanade and install flood gates; build a floodwall and flood gates along edge of Tower Gardens; x Raise embankments at Museum Gardens; and x Replace the existing gates at Marygate and Earlsborough Terrace; raise the wall at Earlsborough Terrace; increase the height of the flood gate at Lendal Bridge; raise the wall at Wellington Row and North Street and inside the car park at Park Inn; and raise the wall at St George’s Field car park and access ramp.
16 3.2.6 Osbaldwick and Tang Hall Beck community x Construct new flood walls or gates to provide additional protection at various locations along both watercourses.
3.2.7 Clementhorpe community x Raise the access to the bridge across the floodplain and install an access ramp; x Construct new embankments downstream of Rowntrees Park to reduce the risk of flooding to Butcher Terrace, Terry Street and Reginald Grove; x Increase the height of existing walls at Terry Avenue and Postern Close; the threshold of the steps at Dukes Wharfe, the south defence wall and the road way platform entrance; the existing defences at Lower Ebor Street and extend the flood wall to near Bewlay Street and raise the height of the flood defence along Waterfront House on Terry Avenue; and x Construct new flood walls or gates to the rear of Waterfront House; build a new demountable barrier across Clementhorpe Street which could connect into the existing defences.
3.2.8 Fulford and Germany Beck community x Increase pumping capacity and improve the outfall of Tunnel Drain; x Raise a section of Fordland Road where it crosses Germany Beck and join this into the new Germany Beck spine road to hold back water from the River Ouse; x Increase the height of the retaining walls at Grange Garth and Alma Terrace running along New Walk and include new flood gates; and x Build a new floodwall on St Oswald’s Road; extend the existing floodwall on both sides of Landing Lane and extend the existing floodwall on Fordlands Road.
3.2.9 Bishopthorpe community x Raise a section of Bishopthorpe Road to remove an existing depression where water gathers; x Build a new flood wall around the Dell, joining the high ground at Bishopthorpe Palace and raising the bottom end of Chantry Lane; install a penstock to the Chantry Lane manhole; and x Install a small pumping station at the end of Chantry Lane to pump surface water over the new flood wall.
3.2.10 Naburn and Acaster Malbis community x Raise land at Moor Lane; x Build a new defence along the river bank adjacent to Front Street and Maypole Grove; x Install a new pumping station in the low spot on the B1222 road; x Create storage area in the field that’s west of the railway line on Howden Dyke; and x Consider modifying Naburn Weir if it is seen to reduce river levels.
17 4. Catchment Scale Measures to Reduce Flood Risk in York
The previous chapter outlined the possibilities for shorter-term work over the next few years that will improve the standard of protection to people, properties and infrastructure from flooding, once the water is already in York. This document is designed to consider strategic, catchment- scale measures, that would be implemented upstream of the city, by which to slow the flow of water and ultimately lower water levels through York during a flood event. Upstream of Naburn weir there are approximately 3,500km2 of catchment, contributing flows to the Rivers Ouse. This presents a significant area of land where opportunities may exist to reduce the river flows reaching York (and further downstream to Selby) during flood events by:
x managing rainfall running off the land; x slowing the speed with which it arrives in the rivers; x storing excess river flows on the land; and x artificially lowering water levels in the rivers. These measures have been considered in the following chapters. Section 4.1 discusses the hydrology of the upstream Ouse catchment identifying important characteristics that need to be considered if catchment-scale measures to manage the flood risk in York are implemented.
4.1 Catchment hydrology and synchronicity of flood peaks
Peak flows and volumes in York are a combination of responses from different parts of the River Ouse catchment that respond to rainfall in different ways. It is important to understand the hydrology of these catchments, both individually and collectively, to ensure that any strategic measures are targeted where they will have the greatest positive impact and will not increase flood risk downstream. If a co-ordinated, catchment scale approach is not taken there is the danger that the timing of flood peaks could be synchronised and flood risk could actually be increased downstream as a result of interventions.
4.1.1 Ouse Catchment Upstream of York, the Ouse catchment comprises of three main tributaries: the Rivers Swale, Ure and Nidd. Figure 4.1 illustrates the locations of river gauging stations near their downstream extents. Flow data from these gauges has been analysed for four recent flood events to assess the contribution of the tributaries to the cumulative flow arriving at York. The four flood events analysed were: November 2000, January 2005, September 2012 and December 2015. Table 4.1 presents the peak flows for each of the catchments and the times when the peak was recorded. Figure 4.2 illustrates the tributary flood hydrographs for the Boxing Day 2015 flood event by way of an example.
18 Figure 4.1: Location of river gauging stations reviewed
November 2000 January 2005 September 2012 December 2015 Catchment Date & Date & Date & Date & Q* Q* Q* Q* & Gauge Time Time Time Time Nidd at 3rd at 8th at 26th at 26th at 208 85 184 247 Skip Bridge 12:45 10:15 03:45 22:30 Ure at 3rd at 8th at 26th at 26th at 397 484 547 489 Westwick 10:30 19:30 06:00 23:30 Swale at rd th th th 3 at 9 at 27 at 27 at Crakehill 213 157 239 214 13:15 09:15 02:00 04:45 Topcliffe Ouse at 4th at 9th at 27th at 28th at 534 398 496 508 Skelton 00:15 08:45 05:45 00:45 Table 4.1: River gauge data * Peak flow (m3/s)
19 Figure 4.2: Tributary hydrographs resulting in the Boxing Day 2015 flood event
Table 4.1 shows that the different sub-catchments all peak within hours of each other and typically contribute flows between 200m3/s and 500m3/s. With the response times being so close, it is vital that any proposed measures that slow the flow of water and delay the hydrograph, do not cause the flood peaks to coincide with one another, thereby potentially exacerbating flooding downstream in York. The individual catchments are discussed below.
4.1.2 River Nidd The four events analysed in Table 4.1 indicate that the River Nidd catchment contributes approximately 200m3/s to flows through York. The River Nidd is quick to respond to rainfall inputs through the upstream sections where the watercourse flows off the foothills of the Yorkshire Dales. However, through the lower reaches, particularly downstream of Harrogate, were the River Nidd flows through flatter valleys the watercourse is much more sinuous and as such has long travel times through the lower part of the catchment. Despite the lengthened travel times in these downstream areas, Table 4.1 indicates that the Nidd typically is the first of the three sub-catchments to reach its peak flow (the exception being November 2000 when it was the second to peak).
On its own, the River Nidd would not pose a flood risk to York: flows of 200m3/s are not sufficient to cause flooding from the Ouse in York.
4.1.3 River Ure The River Ure is the first of the three catchments to start responding to rainfall, but is typically second to reach its peak after the River Nidd, a function of it having a larger catchment area. The exception was in 2000 when it was the first of the three sub-catchments to reach its peak flow. However, it must be noted that there is relatively little difference between the Nidd and Ure catchments in terms of the time the peak occurs: in December 2015 there was only one
20 hour separating the peak flows on the two catchments, and approximately two hours in November 2000 and September 2012.
The River Ure also contributes the greatest flows to the River Ouse through York. Table 4.1 indicates that peak flows from the Ure close to the confluence with the River Ouse are of the order of 500m3/s, significantly more than either the Nidd or the Swale.
4.1.4 River Swale The River Swale has the largest catchment area and is the last of the three sub-catchments to reach its peak flow. In the headwaters the Swale responds quite quickly to rainfall inputs but when it gets towards Catterick the gradient of the river significantly reduces causing a very long, drawn out peak down to Crakehill, as seen in Figure 4.2. This means that despite having the largest catchment area, it does not contribute the largest flows. The peak at Crakehill can often be around the same time as, or even sometimes later than, the peak in York.
The Swale typically contributes around 200m3/s, which like the Nidd, would not cause a problem in York by itself.
4.1.5 Take away points As the slowest sub-catchment to peak, further delaying the Swale hydrograph ought to benefit York and other places further downstream. The Ure and Nidd catchments are more complicated. In Figure 4.2, both rivers exhibit ‘peaky’ hydrographs which suggests that there are opportunities to implement measures to slow their response, attenuate peak flows, and give longer, flatter hydrographs. However, this may risk synchronising the peak flows and exacerbating flooding downstream in York. Modelling would be required to give confidence that any measures considered for the Ure and Nidd would not increase the downstream flood risk. That is not to say that measures to slow the flow in the Ure and Nidd catchments would not bring benefits in terms of flood risk locally, rather a catchment-wide approach would be required to ensure the cumulative impact of these measures did not exacerbate the downstream flood risk in York.
If there is the opportunity to provide storage downstream of the Swale/Ure confluence, this problem may be avoided and it would give more opportunity for the Nidd flows to reduce ahead of the input from the Swale and Ure catchments. The relatively lower flow contributions of the Nidd and Swale suggest that provision of storage on these tributaries would probably not bring significant benefits to York and any storage would need to account for the River Ure inputs.
The contribution of volumes from the Ordinary Watercourse parts of each catchment also needs to be considered. In some locations, such as Swaledale, this may be significant. This in turn influences the most effective and efficient locations for water storage and attenuation measures, for example multiple small scale interventions in tributary headwaters or fewer larger measures on the downstream main river network.
4.1.6 River Foss The other key tributary to the River Ouse is the River Foss which meets the Ouse in York (Figure 4.1). There is only one gauging station on the River Foss: at Huntingdon which is upstream of the two main tributaries: Tang Hall Beck and Osbaldwick Beck. Therefore it is not been possible to undertake a similar review of flood events on the Foss catchment based on observed gauge data.
21 5. Upstream Storage
The River Ouse catchment upstream of York delivered over 500m3/s of water to the city in December 2015 and the 1% AEP event is estimated to be of the order of 600m3/s. Upstream storage to remove a significant proportion of this water, and thereby lower the downstream flow and consequently water levels, presents significant opportunities to manage flood risk in York.
The catchment upstream of York provides the opportunity to store flood water before it gets to York. By storing and controlling the release of water from the upstream catchment, the flood peak in York can be delayed and the volume of water that reaches the City of York and consequently the water levels through York, reduced. This section of the report considers the volume of water that might need to be stored to benefit York and reviews what opportunities exist to provide additional storage within the upstream catchments.
5.1 How much water might need to be stored?
A high-level assessment of the volumes of flood water in central York has been undertaken using the York Detailed Hydraulic Model (Table 5.1).
Volume (m3) of water requiring storage to: Keep water levels AEP (%) Keep the River Prevent property below the existing Ouse in bank flooding flood defences 10 91,050,651 77,771,914 - 5 102,752,330 89,381,667 2 119,241,903 105,763,609 - 1.33 127,042,708 113,521,184 1 132,307,740 118,756,375 - 0.5 147,518,554 133,891,941 1,733,270 1 + 20% 167,279,638 153,564,936 9,472,696 climate change 0.1 184,940,200 171,158,227 19,224,988 Table 5.1: Upstream storage volumes required for level reductions at York
The numbers in Table 5.1 are extremely large. To put them into some context: over 9 million m3 of storage is required to accommodate the potential impacts of climate change and maintain future water levels below the existing flood defences through York. Clifton Ings, the existing storage area in York has a volume of over 2.4 million m3, therefore another four storage areas the size of Clifton Ings would be required to provide this volume of storage. Another way of looking at this volume is it is the equivalent of 3,789 Olympic sized swimming pools.
Table 5.2 shows the volume of water that would have to be stored on the River Foss at different return periods in order to a) keep it in bank; and b) prevent property flooding. It also includes the Boxing Day flood for reference: a storage area about the size of Clifton Ings would have been required to keep the Foss in bank.
22 Volume of water required to Volume of water required to AEP Flood Event (%) be stored to keep the Foss be stored to prevent in bank (m3) property flooding (m3) 10 386,300 92,400 5 611,800 246,900 2 954,500 518,000 1.33 1,124,500 658,800 1 1,230,600 748,600 0.5 1,638,800 1,101,600 1 + 20% climate change 1,815,200 1,245,00 26th December 2015 2,199,900 1,539,700 Table 5.2: River Foss Storage Volumes
The rest of this chapter explores potential means by which these volumes of storage might be able to be provided to further protect York.
5.2 Water Supply Reservoirs
There are a number of reservoirs in the upstream catchments which are operated by Yorkshire Water and City of York Council (Table 5.3 and Figure 5.1). There have been calls for water companies to drawdown their reservoirs in advance of a flood, so that they have an increased capacity to store flood water, thereby protecting communities further downstream.
Catchment Waterbody Nidd Angram Reservoir Nidd Scar House Reservoir Nidd Gouthwaite Reservoir Ure Semerwater Ure Leighton Reservoir Ure Roundhill Reservoir Foss Oulston Reservoir Table 5.3: Existing Upstream Storage Reservoirs There are six water supply reservoirs within the SUNO catchment: located within the Nidd and Ure sub-catchments (there are no water supply reservoirs on the River Swale) operated by Yorkshire Water. The reservoir located in the upper reaches of the Foss catchment is operated by City of York Council.
If the operating rules of these reservoirs were to be changed, the reservoirs could either be permanently kept at a reduced top water level to provide additional storage volume for flood events, or the reservoir could be drawn down in advance of a flood event. This could potentially contribute to reducing flood risk in areas downstream of the reservoirs.
However, their location within the catchment also limits their ability to provide flood protection. As seen in Figure 5.1, these seven reservoirs are located high up in the catchment, a long way from York. Consequently there is a relatively small proportion of the catchment from which the reservoirs are able to store water. Those six reservoirs in the upstream Ouse catchment have a combined upstream catchment area of approximately 200km2, which represents 6% of the catchment upstream of York. The upstream area of Oulston Reservoir comprises less than 2%
23 of the total Foss catchment area. Consequently even if these reservoirs were utilised to store excess flood flows, there is still 95% of the catchment contributing flows to York.
If the reservoir operating rules were permanently changed for flood risk management, there would be a detrimental impact upon water supply. The alternative would be to drawdown the reservoir in advance of a flood event. The problem with this being that because the reservoirs are located right at the top of the catchments, there would be a very short warning period in advance of a flood in which to drawdown water levels. This would create onerous operational procedures to safely drawdown the reservoir and would likely be impractical.
It is anticipated if operating rules could be changed to allow storage of some additional volume, the existing storage reservoirs may be able to contribute to a localised reduction in flood risk immediately downstream. However, these reservoirs within the upstream catchment are not considered to have potential to benefit water levels through York during a flood event.
24 Figure 5.1: Location of existing reservoirs upstream of York and existing washlands
25 5.3 Washlands / Storage Areas
Washlands are areas of land that are designed to flood and store excess water. They can be “off-line” or “online”. Online storage involves creating a barrier across the river and floodplain. This enables the amount of water passing downstream to be controlled and can reduce flood risk downstream. (Informal online storage can occur from the build-up of water behind bridges or culverts). There are no existing online storage areas within the upstream catchment of either the River Ouse or River Foss.
Off-line storage areas are adjacent to the river channel, and are designed to start filling with water when the river level reaches a certain threshold. This allows floodwaters to be held back and reduces the flow passing downstream and so reduces downstream water levels.
To be most effective, the use of online and off-line storage requires the use of land that does not currently flood in order to create additional storage within the catchment. Alternatively, it may require land that currently floods to remain under water for a longer period of time. Additional work may be required to enable areas of land that currently flood to hold more water, by building embankments to retain the water.
Off-line storage can be controlled or uncontrolled. Controlled storage areas, such as Clifton Ings (Figure 3.1 and 5.1), have inlet and outlet structures. These can be operated to fill and empty the storage area in accordance with defined rules. Uncontrolled storage areas, such as Cawood-Wistow Lordship Ings, fill via spillways within the embankments; there is no active control over when the storage fills. The outflow from uncontrolled storage can be by gravity discharge or pumping.
5.4 Existing Washlands
Extensive washlands exist within the SUNO catchment both upstream and downstream of York (Figure 5.1). There are no existing washlands within the River Foss catchment.
There could be potential to modify some of these washlands to store more water or operate in a different way so as to lower peak water levels downstream. These modifications may include alterations to existing embankments to allow the washlands to flood at a different peak level and could present significant benefits to York by reducing water levels through the city.
Table 5.4 presents a selection of the existing washlands upstream of York on the River Ouse up to the Swale-Ure confluence, which may have the potential for modification to increase upstream storage. Table 5.4 indicates the area of the washlands, the volume of water that could be stored in them and also states whether or not the washlands are classified as being functional or non-functional.
26 Catchment Washland Name Current Status Area (km2) Volume (m3) Ouse Clifton Ings Functional 0.70 2,454,200 Ouse Linton Ings Functional 1.23 3,085,000 Swale Myton Ings Functional 1.91 3,040,000 Swale The Holmes Functional 2.22 3,544,000 Ouse Beningbrough Ings Non-Functional 0.22 238,480 Ouse Ferry Ings Non-Functional 0.34 375,650 Ouse Town Ings Non-Functional 0.32 478,800 Table 5.4: Existing washlands
Figure 5.2 shows Myton Ings on the Swale in operation during the November 2000 floods.
Figure 5.2: Myton Ings (River Swale) during November 2000 event, looking upstream
27 5.5 Potential Washlands
As an alternative to, or in combination with, modifying existing washlands, new washlands could potentially be created to provide additional protection to York.
5.5.1 SUNO Catchment As discussed in Section 4.1, additional storage somewhere between the confluence of the Rivers Swale and Ure, and the village of Skelton would provide benefits to York, an area which already contains 11 washlands including Myton Ings, Linton Ings and Fairfield Manor some of which are functional and some non-functional. Section 4.1 also highlighted that storing the Swale and Ure flows whilst allowing those of the Nidd to pass would be preferential.
Myton Ings and Linton Ings are existing washlands at the up-, and downstream extents of the river reach between the Swale-Ure confluence and the confluence with the Nidd (Figure 5.1). Whilst modification of these washlands could present opportunities to lower downstream water levels, so could a new washland in-between the two Ings. Locating a washland in this area would allow the Nidd flows to pass through York ahead of those of the Swale and Ure and have the potential to lower water levels in York. Further work could be undertaken to determine whether there is potential for a large storage area to be located on the Ouse downstream of the Swale/Ure confluence but upstream of the confluence of the River Nidd and River Ouse.
A number of alternative locations for upstream online storage exist within the SUNO catchment, away from the area downstream of the Swale/Ure confluence. In general, areas where the width of the floodplain is reduced, either naturally by high ground, or artificially by existing roads, railways or bridges were identified as potentially suitable locations. Sites are located adjacent to the A1 on the River Nidd, at Morton-on-Swale and Maunby on the River Swale, and at Roecliffe on the River Ure.
5.5.2 Foss Catchment The potential for new washlands within the Foss catchment is limited. The floodplain is constrained through the lower reaches and there are limited opportunities to develop areas of upstream storage on the River Foss or tributaries without significant excavation. Two potential offline storage areas were identified as part of a study on Westfield Beck however, neither of the sites have a natural storage capacity and would require excavating.
Whilst no specific areas have been highlighted to create online storage, there are a number of raised embankments that pass over the River Foss and its tributaries at Westfield Beck, Tang Hall Beck and Osbaldwick Beck. These could be re-purposed for use in flood management in these locations. Further work into the potential for reusing this infrastructure is required to determine whether creating online storage in this way is viable.
The latest flood mapping from the York Detailed Model indicates that the York-Scarborough railway line at Strensall restricts the flow passing downstream on the River Foss. Towthorpe Road bridge, 1.3km further downstream and the A1237 ring road also restrict downstream flows (Figure 5.3). Where Tang Hall Beck passes under Stockton Lane and Osbaldwick Beck passes under the dismantled railway line and A64 could present opportunities to throttle flows restricting the pass forward flow to the urban areas further downstream.
28 Figure 5.3: Structures restricting the Foss flow downstream
5.6 Recommendations
Flood defences through York cannot be continually raised higher, it will also be necessary to reduce the volume of water that reaches the city during extreme flood events. Based on the high level modelling that has been undertaken, approximately 9.5 million m3 of storage (another four Clifton Ings or 3,789 Olympic sized swimming pools) is required within the Ouse catchment upstream of York simply to maintain future water levels below the height of the existing defences.
The first step in taking this forward would be a high level economic appraisal to assess if providing upstream storage is an economically viable option which warrants further consideration. Based on the economic assessment that was undertaken as part of the 5-year Plan, the total Do Minimum (Present Value) damages from those flood cells bordering the Ouse, i.e. excluding the Foss, are £146 million. A large proportion of these damages would be used to justify the hard defences through York being proposed as part of the 5-year Plan.
29 If it were to be assumed that the 5-year Plan provides defences through York providing a 1% AEP standard of protection, the residual (Present Value) damages available would be almost £70 million. Assuming it was possible to construct four new washlands of comparable size to Clifton Ings (2.4m3) they might have a ballpark cost of £40 million. On the basis of which, it is recommended that the Environment Agency undertake further investigations into the viability of providing storage upstream of York on the Ouse. This work needs to be undertaken with an appreciation of ongoing the 5-year Plan.
Secondly, there is a need to better understand the impact that the existing washlands exert on water levels through York and subsequently research opportunities to use these washlands to lower water levels through York. A similar exercise is currently being undertaken looking at the washlands downstream of York.
The York Detailed Model, and the forthcoming Washlands model do not extend further upstream than Skelton on the River Ouse. A broad-scale 1d routing model would be capable of representing the existing storage areas and simulating the flow downstream of them, within the reach of interest. A similar model was produced for the Ouse Catchment Flood Management Plan in 2010 however this would require updating.
The model would not need to be combined with the existing York Detailed Hydraulic Model, nor the York Washlands Model currently in production. Hydrographs could be extracted from the proposed broad-scale 1d routing model at Skelton, and subsequently run through the York Detailed Model to evaluate the impact on water levels through York. The gauging stations at Crakehill, Westwick and Hunsigore on the Swale, Ure and Nidd respectively could form the upstream limits of the routing model. These gauging stations would facilitate the use of observed flood event data by which to verify the model outputs. The model could subsequently be used to:
x simulate a “no washlands” scenario whereby all of the washlands are removed from the model to evaluate the benefit they currently provide to York; x assess if the operation of the existing washlands could be adjusted to optimise the benefits realised in York; and x appraise the potential to create new storage areas. Due to the size of the SUNO catchment work needs to be undertaken to establish a delivery partnership with all relevant stakeholders (including the Environment Agency, City of York Council and North Yorkshire County Council) in order to deliver upstream storage.
5.7 Constraints, risks and opportunities
The provision of additional upstream storage could present a significant opportunity to lower water levels through York during flood events. However, there would also need to be a consideration of other factors that could present constraints, risks or opportunities other than flood risk. These might include:
x Existing land-use; the Vale of York, upstream of Skelton is productive agricultural land and there may be reasonable opposition to creating additional flood storage areas that would result in greater damage to crops and land; x Land-use change; it could be argued that creating new storage areas would change the current land use of an area. However, as evidenced by Figures 5.2 and 5.4, the entire
30 Ouse valley is operating as a floodplain during major flood events therefore the land use during a flood would be effectively the same. The main changes would be that the existing embankments were modified to adjust when the land flooded and potentially for how long it was flooded. The effects of that greater water depth/duration would need to be assed to determine whether these would have any significant effect on current land management practices; x Storing more water and/or for longer periods may affect the rate of silt deposition. This may have positive, e.g. increased fertility or negative effects on the land use practices in those locations; x Optimising the washlands to provide protection during extreme flood events, could cause more water to be passed forward during lower magnitude events, thereby raising water levels during these flood events; x Structures could be put in place to control, or limit, the pass forward flow of the river thereby maximising the storage available in the washlands such as is the case with the “regulators” which limit the flow into the River Don; x A new washland in excess of 25,000m3 may fall under the remit of the Reservoir Act (this threshold could potentially reduce to 10,000m3 in the future) which would be associated with significant costs both in terms of construction and the ongoing inspection and maintenance of the reservoir; and x A number of the existing washlands are designated for their biodiversity value. The effects of increased flood water depth and/or duration upon these sites’ designated features would need to be considered. Other environmental and heritage conditions may present constraints or opportunities.
31 River Ouse direction of flow
Poppleton
Skelton Bridge
Figure 5.4: River Ouse, November 2000 looking upstream of York
32 6. Natural Flood Management
Natural Flood Management (NFM) is a broad term encompassing a variety of approaches that replicate or work with natural processes within the catchment including: afforestation, grip- blocking and re-connecting the channel and floodplain. NFM is a catchment wide approach that involves working with natural processes to manage sources and pathways of flooding. It involves balancing and integrating the restoration of natural processes with existing land uses including the restoration, enhancement and alteration of natural features and characteristics. NFM measures are focussed on managing flooding within the catchment.
Installation of NFM measures within a catchment aims to reduce the amount of runoff entering watercourses and/or improve the ability of watercourses and their floodplains to manage flood flows. This can be achieved by storing more water on land and slowing the flow of water overland or instream (Figure 6.1). The desired effect of implementing NFM measures is to reduce the downstream flood peak and/or delay and elongate the flood peak downstream. NFM measures can decrease the volume of water entering the watercourse, reducing the scale and therefore impact of the flood and can increase the amount of time to prepare for a flood event. NFM can deliver multiple benefits across the catchment including managing flood risk, habitat creation, water quality improvements, carbon storage, amenity, recreation, and access.
Figure 6.1: The impact of land management on flood risk. NFM seeks to reinstate / emulate the catchment on the right Source: Green Alliance (2016)
NFM has been proven a viable technique for reducing runoff in small catchments for some smaller scale flood events, slowing the flow and restoring floodplain connectivity, and thus reducing flood risk downstream. It is, on the whole, a low cost, largely un-engineered solution
33 that provides an opportunity to deliver multiple social and environmental benefits. However, despite the success of pilot schemes (see Appendix A) NFM has yet to be widely implemented throughout the UK and there is still a high degree of uncertainty surrounding the technique. The science behind NFM to date is largely based on modelling and, whilst it is increasing as pilot catchments are set up, empirical evidence is limited. Where observed data is available, the results are encouraging, particularly for small catchments (<10km2). However, there are limitations associated with scaling up the impacts, the scale of measures required across all significant tributaries of a larger catchment and the time lag of years or decades for some measures such as tree planting to become fully operational being two examples. Added to the gaps in our knowledge of NFM impacts on flood risk, there are often barriers to implementation of measures on the ground including: funding, land take/ownership, and maintenance.
A Literature Review outlining the research carried out on NFM to date is included in Appendix A. The review outlines the types of NFM measures, demonstrates practical application of NFM through illustrative case studies and discusses the potential barriers and limitations in more detail.
6.1 Surface Water management and SuDS
Whilst not strictly included under the NFM ‘umbrella’, Sustainable Drainage Systems (SuDS) should be considered to deal with surface water management throughout the catchment. Runoff, particularly in urban areas where impermeable areas are high, can contribute to flooding during storm events by increasing the amount and speed of runoff to watercourses. Where rainfall is no longer able to infiltrate into the ground, impermeable areas such as roads may form flow paths and increase overland flow levels. SuDS are an effective way to implement drainage in urbanised areas that mimic more natural drainage and can provide additional benefits such as improving water quality, recreation, amenity and access.
As the Lead Local Flood Authority, City of York Council is the responsible body for planning within York and the surrounding area. In conjunction with the development of the Strategic Flood Risk Assessment, City of York Council are in the process of producing a Drainage Guidance Document for developers that outlines how future development should consider drainage and SuDS in order to ensure development plays its role in reducing flood risk.
North Yorkshire County Council is the Lead Local Flood Authority for the remainder of the SUNO catchment and has produced ‘SuDS Design Guidance’ to be used by developers and the Local Planning Authorities responsible for development control in these areas. The guidance note provides direction for the successful implementation of SuDS to ensure appropriate management of surface water.
Given the relatively little urban development (existing or proposed) in the SUNO catchment, implementation of SuDS within the upper catchment is likely to have little or no impact on flood risk in York. Implementation of SuDS may be more beneficial to York where implemented in the Foss catchment. However, in both cases development control that stipulates the use of SuDS will provide localised benefits and should continue to be enforced throughout the catchment. SuDS in the urban area/s has the potential to reduce the extent of sewer flooding by reducing rainwater ingress into York’s sewer network and thus relieve some capacity pressure where fluvial flood water would not overwhelm the sewer system.
34 6.2 NFM in the River Foss Catchment
Following the flooding of York in December 2015 Floods Minister Rory Stewart identified the need to look at NFM measures alongside hard engineering options in the city centre. In response, a study into the potential opportunities for NFM within the River Foss catchment has been undertaken to inform this report. The Foss NFM Study identifies potential areas within the catchment where NFM measures could be implemented and then uses modelling to determine the effectiveness of the measures on reducing flood risk downstream.
The Foss NFM Study is included in Appendix B and the main findings are summarised within the following section.
6.2.1 Opportunity Mapping and Technical Modelling of NFM As part of the NFM study, high level opportunity mapping was undertaken throughout the River Foss catchment. These opportunities were then modelled to determine the possible impact of implementing NFM measures on the downstream flood hydrograph.
A desk based assessment of the Foss catchment was undertaken using LiDAR, aerial imagery and OS mapping. Areas where there was potential for NFM measures to be implemented were mapped using GIS software to create a long list of 392 NFM opportunities within the Foss catchment. Once the long list of options was complete, a ranking exercise was undertaken to create a short list of opportunities.
The opportunity and constraints ranking was tailored to each feature to ensure that the ranking was representative and appropriate for the measure identified. This provided a simple comparison of viability between all of the identified measures. Rankings were allocated based on analysis of aerial imagery and available mapping. Following ranking by opportunity, the features identified were scored according to the constraints on their implementation. The opportunity and constraints scores were then summed to give an overall rank between 1 and 8, where 1 represents measures with good opportunities and low constraints and 8 represents measures with a low opportunity and high constraints. Options that were ranked between 1 and 3 then formed the short list of 244 options which were taken forward for hydraulic modelling. Modelling was undertaken for five different scenarios;
Scenario 1: Measures affecting roughness;
Scenario 2: Scenario 1 plus paleo-channel reconnection measures;
Scenario 3: Scenario 2 plus floodplain reconnection measures;
Scenario 4: Scenario 3 plus offline storage measures; and
Scenario 5: Scenario 4 plus measures affecting runoff.
The models were run for design events of 20%, 5% and 1% AEP.
In all three of the watercourses modelled (Foss, Tang Hall and Osbaldwick Becks), the largest reduction in peak flow and delay in timing was observed for the 20% AEP event when all the NFM measures were implemented. This is the lowest magnitude (i.e. more frequent) event that was modelled. These findings align with the available research. The runoff attenuation features such as offline storage ponds are generally small scale and will fill up earlier and, in the case of
35 roughness based measures, be overwhelmed earlier during higher magnitude events thus limiting their impact.
The modelling undertaken suggests that implementation of NFM throughout the catchment could have a small, positive impact reducing flows in the Foss catchment. Changes affecting roughness and runoff are shown to give the largest impact on peak flow and timing of peak flows across the three modelled watercourses in different design events.
6.2.2 Upper Foss In Scenario 1 the Foss model indicates a reduction in the peak flow of 4% in the 1% AEP event but saw no change in the 5% or 20% AEP events. Scenarios 2, 3 and 4 see minimal change to these results whilst in Scenario 5 the peak flow reduces by 8% in the 1% AEP event and 10% in the 20% AEP event.
From which it could be concluded that the NFM measures implemented in Scenarios 2, 3 and 4 (paleo-channel reconnection, floodplain reconnection and offline storage) do not bring about appreciable flood risk benefits on the Foss. Although, it should be borne in mind that there were only three opportunities identified for paleo-channel reconnection, 17 for floodplain reconnection and three for offline storage. This demonstrates the large number of NFM interventions that are required to exert an impact on the flood hydrograph.
In contrast, there were only 20 opportunities to increase channel roughness (Scenario 1) and yet these achieved a 4% reduction in peak flow in the 1% AEP event indicating that these are much more effective measures. The 1% AEP event experiences a greater reduction in flow than the more frequent AEP events since the majority of measures affecting roughness are those that impact out-of-bank flows (such as buffer strips and riparian tree planting). This means that these measures will only exert an impact on the flood peak when the watercourse comes out of bank and thus shows a greater impact during the large magnitude events.
Scenario 5 not only introduces the largest number of interventions: 88 but also includes the cumulative impacts of the other scenarios, 130 are being simulated in the model. The runoff reduction measures of Scenario 5 are the ones that noticeably reduce the flow in the 20% and 5% AEP events. These measures are those requiring the greatest changes in land use, for example installation of bunds, leaky dams, tree shelter belts, changes to grazing practices, etc., and are the ones with the greatest degree of uncertainty in terms of how they are represented in the model.
The upstream areas of the Foss catchment are predominantly rural with areas of open agricultural grazing land and therefore in principle lend themselves well to methods that reduce, or slow the volume of runoff entering the channel in the upper catchment. Many of the tributaries of the Foss within the rural upstream reaches are low lying artificial drainage ditches and so there is an opportunity to create dams / bunds to temporarily store water on farmland, delaying the rate at which runoff enters the River Foss. Similarly, where the tributaries drain steeper land, bunds or shelter belts can be used to reduce the rate of runoff. The large areas of open grassland, used for grazing, provide an opportunity for afforestation in both the floodplain and wider catchment which will increase infiltration and reduce runoff. However the buy-in of landowners will be critical to their success.
6.2.3 Tang Hall Beck Tang Hall Beck perhaps presents the best opportunity for implementation of a suite of different measures within a discrete area of the catchment. The tributary is predominantly rural and, at 15km2, is of a size where NFM has been shown to be effective at providing flood risk benefits.
36 The model results were similar to those of the Foss in that it was Scenarios 1 and 5 which were found to exert the largest impact reducing peak flows. A key difference being that the roughness measures of Scenario 1 reduced flows across all three AEP events. Scenario 1 simulated 15 measures to achieve a reduction in peak flow of between 2-3%. Scenarios 2, 3, and 4 only introduce two additional measures therefore it is not surprising that there is no change in the flows. Scenario 5 then introduces a further 37 interventions to see flows reduce a further 2-5% compared to Scenario 1. From which it could be concluded that the NFM interventions of Scenario 1 are more effective.
6.2.4 Osbaldwick Beck The modelling results suggest that there are limited benefits from NFM measures on Osbaldwick Beck. No measurable impact on the flood peak was observed for any of the events modelled and the delay in the timing of the peak flow is small. This is however, a result of the relatively few opportunities that were identified and modelled: two measures in Scenario 1 and 13 in Scenario 5.
6.2.5 Limitations It is important to acknowledge that there are limitations to the conclusions that can be drawn from this study, and in particular the modelling. The optioneering carried out was an entirely desk based exercise and no ground truthing or verification of catchment characteristics was carried out. There is the potential that this has resulted in an over, or under, estimation of the number of viable measures that have been identified, and therefore modelled within the catchment.
Additionally, the representation of the model has a number of limitations associated with it which could be influencing the results and whilst based on the best available research at the time of writing, there is still little empirical evidence outlining the impact of NFM measures, and even less that have attempted to quantify those impacts. As such, whilst some of the greatest changes in flows arise from the runoff based measures, this is the area of the modelling where there is the greatest uncertainty.
Across the three watercourses the modelling has shown reductions in the peak flow by a maximum of 10%, when 130 NFM interventions were simulated in the Upper Foss. It could be argued that changes of less than 10% are within the uncertainty parameters of the model given its limitations however, broadly speaking the results are consistent with those of other studies.
6.2.6 Implementation A significant plus point of NFM measures is that they are relatively cheap to implement and maintain for the wider benefits they bring outside of flood risk management. Scale Implementation of NFM throughout the Foss catchment may be progressed via multiple local scale initiatives or via a larger scale/single one. If the likelihood is multiple local scale initiatives then there will be a need to ensure implementation meets prescribed standards to reduce (e.g. de-synchronising peak flows) and not increase flood risk and that comprehensive implementation occurs throughout all identified initiatives. An organisation such as the Environment Agency, to take on that central co-ordination role will therefore be required, as will the agreement and support of key stakeholders such as the Foss IDB and main landowners. The participation of the Foss IDB will be critical, the Environment Agency and partners need to work with the IDB to see how their remit and objectives can be aligned with the ambitions of NFM. This may also help getting buy-in from landowners. The Environment Agency are
37 currently working in partnership with IDBs to deliver Water Framework Directive benefits in other parts of the UK and direction should be taken from these working groups to guide partnership development within the Foss catchment. Timescales Different NFM measures will require different timescales to establish, and accordingly become effective. Upland afforestation for example will require long timescales (decades to centuries) to establish and contribute appreciable flood risk reduction results. Measures such as large wood and grip blocking require shorter timescales to fully establish (2-10 years) and flood storage areas are short term and can provide appreciable benefits immediately following installation.
It is the cumulative impact of all these measures that will improve the overall resilience to flood risk and climate change required through the City of York, and as such none of the measures should be ruled out on the basis of timescales.
NFM Measures
The Opportunity Mapping and associated modelling undertaken as part of the Foss NFM study indicates that a combination of measures throughout the Foss catchment is likely to provide the greatest benefit. Detailed analysis, including ground truthing of all the information gathered as part of this high level study, will enable a greater understanding of the opportunities in these areas and the likely benefits that could be derived from implementation of NFM in the Foss catchment.
The modelling results indicate that the upstream reaches of the River Foss and the Tang Hall Beck are preferential areas for implementation of NFM in the Foss catchment. Implementation of the measures identified here will require changes to catchment management; primarily through implementation of land use measures (for example creating bunds to temporarily store runoff before it enters the watercourse), creation of shelter belts / cross slope tree planting to reduce runoff and afforestation within the floodplain and wider catchment.
Upper Foss
The upstream areas of the Foss catchment are more rural with areas of open agricultural grazing land and therefore lend themselves well to methods that reduce, or slow the volume of runoff entering the channel in the upper catchment through the following measures;
x Many of the tributaries of the Foss within the rural upstream reaches are low lying artificial drainage ditches and so there is an opportunity to create dams / bunds to temporarily store water on farmland, delaying the rate at which runoff enters the River Foss; x Where the tributaries drain steeper land, bunds or shelter belts can be used to reduce the rate of runoff; and x The large areas of open grassland, used for grazing, provide an opportunity for afforestation in both the floodplain and wider catchment which will increase infiltration and reduce runoff.
Tang Hall Beck
Tang Hall Beck presents the best opportunities for implementation of a suite of different measures within discrete areas of the catchment. It is predominantly rural and of the size
38 (15km2) where NFM implementation has been shown to be effective at providing flood risk benefits (Appendix B, Figure 2.1).
Recommendations The main issues encountered with implementing NFM in the upstream reaches are likely to be securing sources of funding and obtaining buy-in from landowners, particularly with regard to farm based initiatives as it is unreasonable to expect landowners to suffer a loss – and both are often interlinked, therefore win-win solutions are required.
While smaller, low cost schemes are unlikely to be eligible for the capital funding available to other types of flood risk schemes, the multiple benefits provided by NFM leave it open to other avenues of funding. Due to the generally self-sustaining, low cost of NFM implementation there is potential for small scale, ad-hoc schemes to be undertaken by individual landowners, trusts or community groups.
Where farm based or land use management measures have been identified in the upper catchment consultation should be undertaken with landowners and bodies such as the Foss IDB at an early stage. It is recommended that the Environment Agency appoint a liaison officer to work with landowners keen to invest in NFM solutions.
To ensure that that the potential negative impacts, for example synchronisation of flood peaks, is minimised, a co-ordinated, centrally led approach is required whereby smaller schemes can be delivered individually as part of the wider initiative.
6.3 SUNO Catchment
Opportunity mapping of potential NFM measures in the SUNO catchment has not been undertaken as part of this study. As discussed in Section 4, the size of the catchment suggests that SUNO lends itself well to the creation of additional upstream storage.
As outlined in the Literature Review (Appendix A) the majority of work carried out to date has been aimed at the small catchment scale. Empirical evidence demonstrating the effectiveness of NFM is generally restricted to catchments around 10km2 in area. Putting aside the measurable effectiveness of NFM in larger catchments - given the need to implement measures in a co-ordinated way – the sheer size of the SUNO catchment means that the scale of implementation required is vast. To provide a tangible example; the NFM work undertaken throughout the Pickering catchment (69km2) required the installation of 352 NFM measures, 37ha of woodland planting and measures implemented on 10 farms, to achieve a standard of protection of approximately 5%. If directly scaled up to the SUNO catchment, (ignoring, for the purposes of this example, the concerns with scaling up referred to in the Literature Review) which at 3,500km2 is nearly 51 times the area of Pickering, this broadly indicates the associated increase in the scale of NFM measures that may be required to have a similar impact. The scale of NFM implementation would need to be even greater if it were to rival the effectiveness of hard defences through York.
Even before undertaking feasibility optioneering, modelling of the benefits and associated economic analysis, implementation on this scale purely to address flood risk in York is unlikely to be realistic. That is not to say that NFM should not be considered in the SUNO catchment – implementation of NFM will provide local benefits in the catchments in which they are implemented, just not to York. The agricultural land use and largely un-forested grassland, as
39 well as large areas of peat moorland, suggest that there are areas that may benefit from implementation of NFM measures.
A Gap Analysis should be undertaken to determine what information is required to assess the potential for implementation of NFM measures within the SUNO catchment. This will enable better targeted delivery of more in-depth studies into the benefits of NFM implementation in certain catchments. A Gap Analysis within the SUNO catchment should include:
x Modelled overland flow paths – to identify key locations where runoff could be addressed at source through land use management measures; x Desk based catchment appraisal – using mapping and aerial imagery to identify the land use within the catchment and to identify types of NFM opportunities that there might be within the SUNO catchments; and x Review of existing studies / schemes – including recent work by the Environment Agency to identify pilot NFM catchments. This will enable an integrated approach to future management within the catchment. Further work, similar to the modelling carried out on the River Foss (Appendix B), could be carried out to ensure that implementation of NFM in the contributing catchments does not synchronise flood peaks as this could have the effect of increasing flood risk downstream.
An additional point to note is that the location for many NFM measures would be on or near Ordinary Watercourses which will require liaison with the relevant Lead Local Flood Authority. Any measures in or near an Internal Drainage District will similarly require liaison with the relevant IDB.
6.4 Application of Natural Flood Management
Adoption of NFM measures in the Foss could be effective to help reduce peak flows in combination with other options for flood risk management. NFM measures would be targeted primarily at the upstream reaches where the land use is more rural and presents greater opportunity for a broad suite of measures. Changes to catchment management will be important to store water in fields or uplands. Tang Hall Beck presents opportunities for implementation of NFM. The construction of debris dams or reconnecting floodplains would locally store water or delay transmission of flows to the main River Foss.
The size and predominantly rural land use across the SUNO catchment provides a number of opportunities where NFM measures could be adopted. In isolation, each NFM measure will provide a small attenuation and flood risk benefit, alongside additional benefits such as biodiversity, water quality and amenity. However, given the scale and largely agricultural nature of the SUNO catchment there are a number of opportunities which could be widely used to hold water in the contributing catchments and reduce volumes and peak flows passing downstream which would have a greater cumulative effect.
The huge catchment that drains the River Ouse results in large volumes of water flowing through the City of York. NFM measures alone will not be able to reduce this significantly. Catchment wide NFM does however present the opportunity to increase climate change resilience and provide additional benefits. In addition, the often low capital cost and the potential for implementation using low skilled labour means that NFM it can be applied by voluntary groups in ways that hard defences cannot.
40 Striking a balance between investment in NFM and hard defences will be important. Despite the obvious benefits, and lack of dis-benefits, of NFM it will not resolve flooding in York on its own. Resources should be allocated proportionately and flood risk resources should be concentrated on how and where flood risk through the town can be significantly reduced, whilst developing NFM solutions in parallel.
6.5 Recommendations
The NFM assessment of the River Foss catchment that has been undertaken has demonstrated that:
x There are many potential opportunities to implement NFM measures in the Upper Foss and Tang Hall Beck catchments however, opportunities appear limited within the Osbaldwick Beck catchment; x The implementation of NFM measures in these catchments has the potential to exert a small reduction in peak flows, and delay the timing of the hydrograph peaks in the River Foss catchment based on a routing model produced for this study; x Modelling suggests that the implementation of runoff based measures, particularly farm scale land use management, and roughness based measures, in particular riparian planting (buffer strips or tree planting) and installation of Large Woody Debris, should be targeted in the Upper Foss catchment and on Tang Hall Beck; and x NFM implementation in the SUNO catchment is unlikely to result in tangible flood risk benefits in York. In order to refine the findings of the modelling a number of suggestions have been outlined in the following section.
6.5.1 Further Technical Studies x Ground truthing of the Foss catchment is required to confirm catchment characteristics, baseline assumptions and to verify the feasibility of the measures identified; x Following identification of potential locations for NFM implementation, a 1D / 2D model should be developed based on channel surveys. This will enable development of a more detailed model of the watercourse and enable more explicit modelling of measures in the floodplain such as removal of embankments and floodplain tree planting. This should provide further details of changes in flood risk within the local area; x Further studies are required throughout the SUNO catchments to better understand current land use management and determine where implementation of NFM measures might be effective. A Gap Analysis should be carried out to determine surface water pathways, land use and the potential NFM measures that could be implemented throughout the catchments; and Implementation of NFM measures within the catchments upstream of York provides a good opportunity to undertake monitoring of the effectiveness of interventions. Studies could be undertaken to determine where monitoring could be carried out in order to gather baseline and measured data following implementation which will feed into the wider evidence base for NFM. Where proposals to implement NFM measures are made, monitoring should be a key component of this process (including developing a robust baseline) to help understand how measures impact flows within the Foss catchment and also to feed into the wider evidence base that is being developed across the country.
41 6.5.2 Funding x The types of measures that have been identified, and the multiple benefits associated with them, can be implemented through utilising a range of funding sources. This provides a broad spectrum of primary drivers - ecosystem services, forestry, water quality and WFD improvements to name a few – for the delivery of which funding can be obtained. In these cases flood risk management will be a secondary, albeit important, benefit; x Work should be undertaken to identify possible funding sources, including FDGiA but also looking at other sources such as WFD funding, the Rural Development Programme, and Agri-Environment funding such as Catchment Sensitive Farming and Countryside Stewardship grants, including the new flood action facilitation fund; and x Some of the on-site measures could provide economic advantages in themselves, and information on these measures could be provided to landowners and users to encourage implementation. Working with a range of Environment Agency departments and other organisations, including the local authorities and Foss IDB, can help understand the multiple benefits of measures such as riparian tree planting and storage and identify a range of potential funding streams. 6.5.3 Partnership Working A multi-agency NFM Implementation Strategy should be developed, integrating NFM ambitions with other objectives such as habitat creation and water quality improvements. It should outline the areas highlighted for further work, including the additional work that needs to be done to realise them, timescales for delivery, and likely funding mechanisms.
x Stakeholder and landowner engagement must be undertaken as early as possible in the process to get buy-in from these parties. The implementation stage should seek to engage stakeholders (local and national) in the decision making process; x The Environment Agency could explore setting up a pilot project in the Foss – perhaps at the farm scale – that would act as a demonstration project for other farms in the area. In conjunction the Environment Agency would need to provide a dedicated liaison officer who would be responsible for landowner engagement including outlining the benefits to farmers and co-ordinating approaches across the catchment; x Partnerships should be explored and work carried out to determine how the different parties (e.g Environment Agency, City of York Council, North Yorkshire County Council, and other Trusts/NGOs) can work together to deliver catchment management; and x Opportunity mapping linked to local community needs and opportunities, Environment Agency and Local Authorities' visions for the area could identify compound benefits where these are delivered together, or where there are specific aspects built into the NFM measures which are deliberately for purposes other than flood regulation. Working with a range of Environment Agency departments and other organisations can help understand the multiple benefits of measures and identify a range of funding streams.
6.6 Constraints, Risks and Opportunities
Implementation of NFM throughout the catchments upstream of York provides an opportunity to increase the resilience of capital defence schemes within the towns and cities in downstream areas. NFM measures are generally low cost, and by their nature generally self-sustaining measures that deliver multiple benefits. This provides an opportunity to tap into multiple
42 sources of funding that might not otherwise be available to schemes of this nature. In addition, NFM provides a unique opportunity to establish a cross-catchment partnership, encourage collaboration between numerous stakeholder groups and collect empirical evidence on a large, as yet unseen, scale.
Whilst there is limited empirical evidence to quantify the benefits of NFM, it has been shown to have a positive impact on flood risk particularly at the smaller catchment scale. Equally, there are very few dis-benefits associated with NFM and most of those that do exist are largely a factor of human land use; for example the risk of Large Woody Debris becoming dislodged and causing blockages at structures (bridges, culverts etc). Even where it cannot be proved that NFM will have a measurable benefit on flood risk, the additional benefits that it provides mean that NFM should be considered and implemented wherever possible.
The main risk to delivery of NFM throughout the catchment is the potential to increase flood risk to the areas downstream through the synchronisation of flood peaks, as discussed in Section 4.1. Further studies will be required to determine the impact of any proposed measures on the timing of the flood peak in each catchment.
There are numerous constraints to delivery – some common limitations to NFM delivery in general, others more specific to the catchments upstream of York. The more general constraints include obtaining funding, timescales of delivery and environmental constraints, all of which are discussed in more detail along with others in the Literature Review in Appendix A. Additionally a vision for the long term maintenance of NFM measures needs to be agreed. This will be dependent on the contribution that NFM makes to delivering the overall flood risk management aim for York, when assessed against other measures such as flood storage and hard defences.
Large scale land use changes such as afforestation and changes to arable, livestock or tillage regimes will also need to be considered against the expectations of landowners and users. The benefits gained by implementing such NFM measures will need to be weighed against any dis- benefits from changing existing land use.
The size of the SUNO catchment means that cross catchment collaboration will be vital to the success of any programme of implementation. Co-ordinating collaboration between a large number of stakeholders, often with conflicting views / priorities, will be one of the largest challenges. Linked to this is the predominantly agricultural nature of the SUNO and Foss catchments, and in the Foss catchment in particular, the dominance in watercourse management by the IDB. It will be integral to the success of any NFM programme within the Foss catchment to work closely with the IDBs to develop a land management strategy that is acceptable to all parties. Obtaining landowner buy in / stakeholder agreement to change the way in which the land is managed in these areas will require extensive and well thought out engagement.
43 7. Engineering Measures
Whilst this document is primarily concerned with catchment based approaches to flood management, it is recognised that hard-engineering measures outside York, could present opportunities to manage the risk of flooding through the city. This section of the report introduces a few of these measures.
7.1 Naburn Weir
Naburn Weir is approximately 8km downstream of York, just downstream of the villages of Naburn and Acaster Malbis. The weir is 55m-long and was first constructed in 1741 to facilitate upstream navigation by raising water levels through York and prevent the tidal cycle influencing water levels. The crest level of the weir was raised over time, probably to enable larger vessels to navigate the river but possibly due to the build-up of silt that would otherwise slowly reduce the size of craft that could operate on the section. The situation today is that under normal conditions the river level through York remains fairly constant and navigable, rising only with rainfall.
A moveable weir arrangement has recently been employed on the River Aire as part of the Leeds Flood Alleviation Scheme. The 90m-long weir can be lowered in flood conditions to reduce river levels and the threat of flooding through Leeds by creating additional channel capacity to store the flood water. Suggestions to modify Naburn Weir in a similar fashion have been proposed many times over the years.
The York Detailed Hydraulic Model has been used to appraise the potential upstream impact of modifying Naburn Weir. It must be noted that the downstream boundary of this model is situated directly downstream of Naburn Weir which may exert an influence on the results. The Washlands model currently being developed would be better placed to undertake this assessment and the Environment Agency may wish to consider re-visiting the findings presented here with the Washlands model.
Three high level scenarios were assessed to determine the impact on upstream water levels:
1. Complete removal of the weir; 2. Permanent lowering of the weir crest to half the current height; and 3. Opening the Lock Gates prior to and throughout a flood event. For these scenarios, the downstream boundary in the model was not tidally influenced. It should be noted that the tidal extent of the Ouse is Naburn Weir and therefore the possible tidal impacts could limit the potential for upstream betterment, or under extreme conditions, may allow the tide to propagate further upstream. Additionally, the uncertainty associated with the downstream boundary is mentioned above.
Scenario 1 is designed to provide an indication of the greatest possible reduction in upstream water levels that could be achieved by completely removing the weir. The model results showed reduced water levels between 0.1m and 0.2m directly upstream of the weir, as far as the A64. However, the benefit decreases to between 0.02m and 0.1m from the A64 up to the centre of York extending as far as the downstream boundary of Clifton Ings.
44 Scenario 2, is perhaps a more realistic situation than scenario 1, where the crest of the weir was reduced to half its current height, that may be able to be achieved through a moveable weir arrangement. However, water levels were only reduced between 0.02m and 0.1m as far as Bishopthorpe and did not extend further upstream into the city.
Scenario 3 is an alternative to scenario 2 where the Lock Gates are opened during a flood event instead of adjusting the height of the weir itself. The model results were effectively the same as those for scenario 2 although the 0.02m to 0.1m reduction in water levels extended as far as the A64.
It may therefore be concluded that modifying Naburn Weir does not offer significant benefit to properties at risk of flooding through York. That is not to say that it does not offer potential benefits for the communities of Naburn and Acaster Malbis, and potentially even Bishopthorpe. However, further modelling would be required before taking this forward as it is anticipated that allowing for the impact of the tide would reduce the reductions seen in water levels and probably raise upstream water levels, particularly during extreme tidal events.
7.1.1 Environmental, Cultural & Heritage Considerations In order to maintain the historical character of York it is not considered practical to completely remove Naburn Weir and in so doing return the river to tidal oscillations upstream of the structure. The removal of the weir would also likely involve the cessation of movement of all but the smallest of navigational traffic at certain times during the lunar cycle.
In addition to navigational problems there are a number of other issues that would need to be reviewed if the weir were removed. An immediate situation following removal will be that water level in the river upstream of the weir will be lowered throughout the flow range and the velocity of flow will increase. This will expose parts of the river that have not been seen for some time and will change the aquatic regime and its associated flora and fauna; habitat and species that have become established may find the new conditions unfavourable for their continued existence, in the short term resulting in elements of biological degradation. At worst this could result in erosion of the bed of the channel, undermining of river walls and banks, and damage to other infrastructure. In extreme cases, the river may attempt to meander, with the risk of damage to adjacent structures or loss of productive agricultural land.
Another issue is that many weirs were constructed in an era when rivers were heavily polluted by industrial waste. As a result, the accumulated sediments that are found upstream of old weirs can be heavily contaminated. These contaminants are relatively safe when left in place, but could be released into the river system if the weir were to be demolished.
In the longer term, retrogression (i.e. erosion) of the river bed upstream may continue. The rate of retrogression will depend on river slope, bed material and flow regime (most bed movement will tend to take place in flood flows). The greatest impact is likely to occur in cases where a very old weir, such as at Naburn, is removed, because the channel regime upstream will have adapted to the flatter water surface slope, and the bed level will have built up due to siltation over the years. With the steepening of the water surface slope due to removal of the weir, this accumulated bed material will tend to be eroded and deposited somewhere downstream.
Given the need for the weir to remain in place to permit navigation to continue, the fact that flooding could be exacerbated by allowing the tide to propagate further upstream and the above concerns, complete removal of Naburn Weir is undesirable.
Since the modelling indicates that opening the lock gates is preferable to actually modifying the weir it is thought to be worthwhile undertaking further modelling, using the Washlands model
45 and allowing for the impact of the tide to determine whether opening the lock gates or restoration of the original sluices at either end of the weir would be sufficient, to provide local benefits for Naburn, Bishophorpe and Acaster Malbis.
7.2 Barrier
Barriers can deal with either fluvial or tidal flooding. The Foss Barrier, for example, prevents the River Ouse backing up the River Foss and causing fluvial flooding in the Foss catchment, whereas a tidal barrier, such as the Hull Barrier, prevents surge tides from travelling up river and causing flooding. A barrier preventing the tide travelling upstream, could increase channel and floodplain capacity to accommodate fluvial flooding.
A barrier upstream of Goole would reduce the risk of tidal flooding to the lower Ouse bringing benefits to those areas where tidal flood risk is the dominant factor i.e. Selby and the agricultural land downstream and on the tidally dominated parts of the Rivers Aire and Wharfe. Preventing the propagation of the tide upstream could also create additional channel (and floodplain) capacity in the lower Ouse to store fluvial flood waters, which may benefit York. However, a barrier may increase the downstream tidal flood risk. For example, a barrier downstream of Selby would prevent the incoming tide travelling up the Ouse to Naburn. This volume of water would have to go elsewhere. In the context of the wider Humber estuary it is unlikely there would be an impact however immediately downstream of the barrier may experience increased water levels.
The Washlands model currently in development ought to be able to provide an insight as to whether increasing channel and floodplain storage downstream of York would provide significant benefits through York in terms of a reduction in extreme water levels. Additionally, the River Humber Strategy is undertaking modelling of the Humber Estuary, although this project only extends as far as Selby.
7.3 Channel Diversion
Flood relief, or by-pass channels are used to divert excess flows in times of flood away from vulnerable areas or bottlenecks. These diversion channels have been implemented at local and large scales.
With regards to York, one possible route could be to the west and south of York, diverting excess flows from the River Ouse at Poppleton and discharge them back into the river downstream of Bishopthorpe, Naburn or into the Fleet watercourse. However, the shallow river gradient through the study reach means that there would not be a significant reduction in water levels, unless the diverted flow can be discharged far enough downstream or to outside the catchment. An alternative might be for the River Foss and other watercourses flowing from the east, including Tang Hall and Osbaldwick Becks, to be diverted to the south and east of York.
There would be significant costs and disruption to existing infrastructure involved with creating a channel diversion around York, and at present it is not known if the channel could provide an adequate standard of protection. An Environment Agency review in 2008 indicated that any diversion channel would be too expensive to economically justify.
46 Nor do opportunities exist for a diversion channel to be constructed to transfer flow outside the catchment. Transferring river flows from the Nidd to the Wharfe, from the Ouse to the Wharfe and from the Wharfe to the Aire have been considered previously and ruled out primarily from topography restrictions that would lead to excessive costs and because the transfers would exacerbate flooding in the receiving catchments.
47 8. Development and rural land use planning
Since the introduction of the Town and Country Planning Act 1947, local planning authorities have had powers to control development in floodplains. Guidance from government sets out how local planning authorities should deal with developments in areas at risk of flooding.
Proposals for developments in areas at risk of flooding are required to produce a Flood Risk Assessment (FRA) to accompany the application for planning permission. The FRA has to quantify the type and scale of risk posed to the development, and show that this risk can be mitigated effectively. The FRA must also show that the development will not increase the risk of flooding elsewhere. Whilst the Environment Agency are consulted on applications in areas at risk of flooding, the final decision on whether the development should be permitted rests with the local planning authority.
8.1 Environment Agency Role
The Environment Agency is a statutory and non-statutory consultee in the planning system in England and Wales. As an advisor to government, the Environment Agency influences and informs planning legislation and planning policy.
Further to this, as an advisor and consultee to regional and local planning authorities, the Environment Agency promotes sustainable development by providing environmental evidence advising on:
x draft strategies; x development plans and other strategic frameworks; x environmental assessments; x monitoring planning applications; and x reporting on environmental performance. On the individual development level, the Environment Agency is a statutory consultee for all developments in Flood Zones 2 and 3. The Environment Agency is consulted for expert technical advice on around 50 higher-risk planning applications and pre-planning enquiries in York per annum, and any developer wishing to develop a site in Flood Zones 2 or 3 should contact the Environment Agency to determine the precise requirements of a FRA.
8.1.1 Spatial Planning Nationally, there is a call for tougher control on development in high flood-risk areas (a view stated particularly clearly in the Pitt Review of the 2007 floods). The Environment Agency’s spatial planning and environmental assessment activities help avoid risks to people from the environment, for example flood risk, and avoid risks to the environment from development, for example from pollution.
Spatial planning provides the longer term perspective to allow the infrastructure requirements of new developments to be met, including environmental infrastructure. It can help to:
x reduce the impacts of climate change;
48 x reduce waste; and x minimise exposure to high levels of air pollution. Where development is essential and cannot avoid environmental harm, spatial planning can ensure that mitigation and compensation occurs. For example, if a development will encroach into space that formerly flooded then an equal or greater area of compensatory water storage should be provided within the site. This means that flood risk elsewhere is not increased. Positive planning should enhance as well as protect the environment, and development can provide an opportunity to install measures that will improve water quality or provide habitat for wildlife.
8.2 City of York Council Role
City of York Council is a statutory consultee in the planning system in England and Wales, and is often referred to as the first tier of Local Government. The role of local councils in the planning process covers an array of responsibilities, which include:
x influencing decisions and policies; x developing city/town/parish plans; x identifying potential sites for affordable housing; and x leading community engagement in implementation projects. 8.2.1 The Local Plan All councils have a statutory duty to produce a Local Plan. A Local Plan sets strategic priorities for the whole city, forms the basis for planning decisions and must be reviewed at regular intervals to keep it up to date. City of York Council is currently working towards a new Local Plan that is fully compliant with the National Planning Policy Framework (NPPF) and other relevant statues.
A Local Plan is important as without adopting a plan development can still happen, however decisions can be made in regard to the NPPF without local people having an opportunity to comment on setting local policies. It is also a critical part of helping to grow York’s economy and create up to 11,000 job opportunities by identifying land for employment uses. In York, approximately 480 hectares of land for housing and 57 hectares of land for employment have been identified.
8.2.2 Control of drainage and surface water in York Compared to fluvial flood risk York does not have high numbers of properties at risk from surface water, but the City of York Surface Water Management Plan identified locations where flood risk has been aggravated by development and highway works. While historically it has been acceptable for surface water from developments and highways to discharge unchecked into drainage systems this is no longer acceptable. PPS25, the NPPF, City of York Council’s Strategic Flood Risk Assessment and the Flood and Water Management Act all require development to incorporate sustainable drainage to manage not only the risk of flooding to the site itself, but also the surrounding area.
SuDS are best done in a ‘soft’ form that creates space for the water, and can add habitat or amenity value to an area. The drainage lagoons around the Vangarde development at Monks Cross in York are an example of this approach. Where space or other constraints prevent this
49 approach then other ‘hard’ engineered options are also available. These usually take the form of an underground storage tank that holds rainwater and then releases it slowly into local drainage systems, at a rate that replicates natural runoff.
SuDS are at their best when designed and integrated in the early stages of any development, and the Flood Risk Management team at the City of York Council takes a very proactive role in development management. They strive to resolve drainage and flood risk design issues at application stage to avoid the need for conditions. Without considering flood risk and drainage as a fundamental element of the design, options to provide sustainable solutions at a late stage of the process are difficult or impossible to achieve. Close working with the Development Management Team is necessary to ensure applications are dealt with appropriately.
The planning approval process does not cover highway works, which, if carried out incorrectly, can have an adverse effect on flood risk. There is a clear requirement in the Flood and Water Management Act for highway authorities to make a contribution towards the achievement of sustainable development and the Flood Risk Management team will work with highway engineers to ensure that there is compliance with this requirement.
Recommendations for action relating to this position can be found in the City of York Surface Water Management Plan.
50 9. Flood Forecasting & Warning
The riverside areas in York are well-used open spaces and footpaths, and future flood alleviation measures in York are unlikely to be purely ‘passive’ structures. When the river rises people need time to move themselves and their property away from the river, and there will be gates through walls and other demountable flood defences that need to be put in place or closed in plenty of time. Controlled washlands upstream of York need an accurate river-level forecast if they are to be operated to best effect, and tidal forecasts need to be factoring into an understanding of risk in the Naburn and Acaster Malbis area.
It is also important to recognise that any flood defence can only reduce risk, not remove it completely. Therefore it is vital that we can accurately predict when river levels might exceed the flood defences and that communities can be warned of the resulting risk. On receipt of a warning people need to know how to react, to take informed action to protect their home and businesses.
9.1 Forecasting
9.1.1 Ouse Forecasting The Environment Agency’s knowledge of a river catchment is based on a combination of historic evidence and computer modelling. As the River Ouse has a long record of flooding, forecasts generated by computer models can be tested for accuracy against historic data.
The forecasting model used to predict levels at the Viking Recorder in central York is considered to have very good overall performance, with accurate forecasts extending 14-16 hours into the future. Confidence in the forecast is greatest when the flood peak has passed the gauges at Hunsingore, Crakehill and Westwick Weir on the rivers Nidd, Swale and Ure respectively, and the greatest risk to accuracy is an error at the Skelton Gauging Station.
The forecast at Naburn Lock is less accurate. The combination of fluvial and tidal influence is not easy to replicate in a computer model, and the historic river-level record at Naburn is not as long as that for central York. The Naburn record may also have been affected by mining subsidence in the area, and the Ouse and Wharfe Washlands modelling project reviewed this issue in early 2017 and is expected to recommend further investigation. The nature of the work required would suit an academic involvement, and the Environment Agency will look for opportunities to work in partnership with a university on this.
It is likely that in the future new forecasting technology will better account for ‘live’ operational actions, such as opening Clifton Ings, as they happen, but even with improved systems there are limits to how far forward in time river-forecasting tools can predict. The river flow and level in York will always be dependent primarily on the inputs from the Swale, Ure and Nidd, and cannot be calculated until the volume of water in these three rivers is known.
If better long-range forecasts for York are to be developed they will be dependent on estimates of the volume of water that will enter the river systems, and this in turn is dependent on rainfall forecasting. The Environment Agency and the Met Office have a working partnership in the Flood Forecasting Centre (FFC). As well as providing 24/7 services now, the FFC has a programme of technical development and aims to get new technology into operational use as quickly as possible. With a large number of properties at risk and a long flood history York is a
51 strong candidate for early adoption of new technology, and local staff will continue to work with Environment Agency flood forecasters and the FFC to ensure the best possible service is provided.
9.1.2 Foss Forecasting The River Foss does not have the same extensive river-level record and flood history as the Ouse, and, as a much smaller catchment, it does not have the same network of upstream monitoring. There are remotely-monitored level gauges on the Foss at Foss Islands Road and on Tang Hall Beck, but the only flow monitoring station is at Huntington. This was installed in 1988 in connection with the construction of the Foss Barrier, and was provided with a forecasting model in 2016. This model is technically good and performs well when assessed against historic data, but has not yet been fully tested with real observed high water levels.
The river-level forecast for Huntington is entirely dependent on rainfall inputs, and so the accuracy of this data forms the greatest potential source of error in the forecast. Errors are most likely to occur when the exact location of intense rainfall cannot be accurately predicted, such as heavy but localised summer storms. The Environment Agency is currently working to install a new river-level gauge upstream of Huntington to provide more real-time information to support the rainfall-based forecast, and is also looking at options for further river-level gauges and rainfall gauges within the catchment.
9.2 Flood Warnings
The Environment Agency operates a 3-tier flood warning service. A Flood Alert is issued when flooding is possible for low-lying land and roads across a wide area. In York there are Flood Alerts for the Upper Ouse and the River Foss. Flood Warnings are much more geographically specific, and cover smaller defined community areas. They are issued when flooding of homes and businesses is expected. Any Flood Warning can be upgraded to a Severe Flood Warning. These are issued only rarely, and indicate that there is a significant and immediate danger to life.
The Environment Agency aims to operate a forecast-led flood warning system, where flood alerts or warnings are issued as soon as forecasts give confidence that flooding will occur in a given area.
9.2.1 Flood Warnings on the Ouse The flood warnings on the Ouse in York have developed over the past 20 years and more in consultation with the Environment Agency’s partners and customers, and are a fairly complex arrangement. Compared to the national average each flood warning covers a relatively small number of properties, and each warning has quite specific ‘trigger’ criteria that have been refined over a number of floods. The majority of the flood warnings on the Ouse, and particularly those that are issued most frequently, have what are known as forecast-led triggers. This means that the warnings are not issued simply when the river reaches a fixed height, but are issued as soon as river level forecasts for York give confidence that flooding will occur. The key gauging station for York is the ‘Viking’ recorder in North Street gardens, but the large flow gauging station at Skelton is also an important reference point.
There is some scope to refine flood warnings on the Ouse in York. The definition of some of the warnings has been lost over time, and the communities they serve are not always clearly described in the warning name. However, caution must be exercised over any change. Many
52 residents and businesses in riverside areas, as well as local authority officers and other officials, are very familiar with the service and use it to trigger specific actions in their own emergency plans. Any flood warning changes by the Environment Agency have to be very well communicated to ensure that these other plans are changed correspondingly. The Environment Agency will continue to work with the community and partner bodies to refine the flood warning service on the Ouse.
9.2.2 Flood Warnings on the Foss Flood warnings on the Foss catchment in York have been less tested than those on the Ouse. The current arrangement was developed in 2005, although there have been some minor amendments since and improvements were made following the floods of 2015. The primary concern for flood warnings in the Foss is to ensure that people are warned in a timely manner but without ‘crying wolf’. Flooding on the Foss is rare, and only a few properties and roads are at risk of regular flooding. These areas need warnings specific to them that can be issued without causing undue concern to people in surrounding areas. However, should there be very heavy local rain or should the Foss Barrier be overwhelmed then a large number of people are at risk of flooding, and this flooding could occur very quickly. If such an event is anticipated then warnings have to be issued as early as possible, often whilst river levels are still low and before the potential peak-level is known.
The lack of river-level forecasting on the Foss in the past meant that warnings were triggered using the best information available, a combination of rainfall information and Ouse levels and forecasts. This meant that forecast situations did not always come to pass, and so lowered customer confidence in the warning service. This situation is improved now that there is a river- level forecast available at Huntington, and following the completion of new flood modelling of the Foss in 2016 the Environment Agency is carrying out a full review of the flood warnings in the Foss catchment. The service could be improved further by additional river-level monitoring stations on the River Foss upstream of Strensall and on Tang Hall Beck in York. Gauging stations do come at a cost and do have to be prioritised in relation to the needs of other communities across the region, and so opportunities to collect information from partners or flood wardens will also be explored.
9.2.3 How Flood Warnings are issued When the Environment Agency issues a flood warning a message is sent to people in that community and to local media and emergency services. This is done by way of a recorded telephone message to landline numbers that the Environment Agency is able to access from telephone service providers, but customers can also receive it on other landlines, mobile phones and computers by registering their details with the Environment Agency. This direct message from the Environment Agency is the primary form of flood warning for most recipients, although flood warnings are also displayed on the internet and through mobile Apps and will usually be repeated by local media and through social media.
Installing sirens to support the telephone message and warn of flooding from the Foss has been proposed following the floods of 2015, and this is an option the Environment Agency is looking into. However, sirens are generally considered to work best in small, steep valleys where all the properties at risk are close together and the danger is of very sudden flooding. The Foss does not fit this profile and so the value of sirens must be carefully considered.
Other methods of reinforcing and supporting the telephone flood warnings include door- knocking and announcements from vehicle-mounted loudhailer units. Both of these activities require a large staff resource to be available at fairly short notice. Over the past few years the Environment Agency has invested in training and organisational change of incident
53 management to allow staff and resources to be moved around the country more easily, but the Environment Agency is a national body with a wide-ranging remit and just over 10,000 staff. It remains a fact that there may not be enough Environment Agency staff in York to carry out all the desirable local actions in a flood, and the resources of emergency services and other partner organisations are likely to also be stretched. The Environment Agency will work with City of York Council and community groups to find ways in which the automated flood warning messages can be reinforced and supported.
9.3 Response to flood warnings
Flood forecasting and flood warnings are only the first stage of the emergency response to a flood, and they are of limited value if the recipients of the warnings don’t know what to do in response. The extent to which people respond to a flood warning depends in large part on their understanding of the flood risk in their area and their previous experience of flooding, and this was clearly illustrated in York on Boxing Day 2015. In very general terms, Ouse-side residents who had flooded before reacted quickly to the flood warnings, and put into action plans as they saw appropriate for their property. People in the Foss catchment who had not experienced flooding before, and often were not aware that they were at flood risk, were not so prepared or proactive in responding to the message.
Since the floods of 2015 the Environment Agency and partners have conducted public events around York, including eight “drop in” meetings held by the Environment Agency, and have participated in several events held by other organisations, such as the University of York. The Environment Agency has partnered with the University of York and York St John University to increase flood awareness among staff and students. The Environment Agency has written to all lettings agencies and social housing providers in York to encourage them to inform their tenants about flood risk and what they can do.
In order to help people make their own plans when flooding is likely, the Environment Agency can include additional information around forecasts and likely areas of risk in the flood warning message. This information is then also displayed and regularly updated on the gov.uk website and the Floodline telephone service. In York these messages always include levels in feet and inches as well as in metres in response to previous requests for this service from customers. The Environment Agency encourages any flood warning recipient to feedback to them on timeliness, accuracy and message content, and will continue to refine and improve this service to best serve the people of York.
9.3.1 Maintaining Awareness and Community Plans The Environment Agency, City of York Council and North Yorkshire County Council are working together to redevelop existing community flood plans and to help communities without plans to develop them. York Centre for Voluntary Service (CVS) is partnering with the Environment Agency to help recruit and train volunteers who can help out in times of flooding, including as flood wardens, and to inform their local communities. The message of flood awareness and resilience is also being spread through schools and community and charitable organisations.
The need to be flood aware extends beyond those whose property may flood. In the December 2015 floods, some residents whose homes were completely unaffected by flood waters were nevertheless cut off from access. Many people under-estimate the risks of driving or walking through even relatively low levels of flood water, which can be polluted, hide dangerous obstacles, or wash pedestrians and cars away. Floods can also cause a loss of essential
54 utilities, cut off access to workplaces, schools and business, and by doing this, cause substantial economic damage. Because of this, the engagement efforts by the Environment Agency and its partners encourages all people, not just those whose property is at direct risk, to develop personal or business flood plans.
Part of maintaining awareness of flood risk could be a regular and visible test of response procedures. This could include ‘open day’ type events at the Foss Barrier to highlight the role that it plays in protecting homes and businesses on the Foss, or a test of the flood warning phone calls and other methods of giving out warnings. If sirens were installed on the Foss then these would need to be regularly tested, and the annual sounding of these where they are used in West Yorkshire does draw attention to flood risk.
9.3.2 Multi-Agency Response The Environment Agency, City of York Council, Yorkshire Water and other national and local government bodies and utility companies all have actions that they take in the event of flooding. These actions are summarised and coordinated through the Multi-Agency Flood Plan, and the parties involved all meet when flooding is expected. This is firstly in the form of the City of York Council-led York Flood Group, and then if river levels are rising further this will become the police-led Tactical (‘Silver’) Command Group for the incident.
The multi-agency response to flooding is well established in York, but the independent York Flood Inquiry following the floods of winter 2015-16 did make a number of recommendations relating to emergency planning and joint working between authorities and the voluntary sector. These recommendations are being followed up by the relevant parties, and progress will be reported by the Environment Agency and City of York Council.
9.4 New technology
The Environment Agency is always looking at how new technology can support its work, with one example in development being a consistent national system to record specific impacts of flooding. Developments in vehicle-tracking technology are helping equipment be better placed and used across the country, and options for the remote monitoring of more sites are being investigated and trialled. In some cases it may be possible for the Environment Agency to work in partnership with local authorities or businesses to gather information on flooding from existing CCTV networks. In other cases more specific new infrastructure will be required, such as cameras and sensors that monitor pumping stations and culverts and allow trash to be cleared from their inlets as required. In York there is a balance to be struck between leaving flood gates open for public access to riverside paths and closing them in time to be sure that they will serve their purpose. Sensors on the gates that allowed remote monitoring of whether the gate is open or closed could help to manage this balance.
55 10. Total Catchment Management
The work that has been discussed in this report arose in response to the Boxing Day 2015 flood event to provide additional protection to the residents of York from the effects of extreme flood events. Hard, engineering flood defences through the City are to be delivered over the next five years whilst in tandem work appraises the possibilities to implement catchment-wide initiatives such as upstream storage and NFM.
The implementation of these two initiatives needs to be considered within the context of:
x Operational management of flooding in the River Ouse catchment; and x River Basin District Management Plans (RBMPs).
Together these can form the concept of Total Catchment Management.
10.1 Flood Risk Management
New, improved flood defences and strategic solutions to flood risk in York form part of a much wider collection of activities and interventions that work collectively to manage flood risk. In addition to the improved defences, upstream catchment management and other water level control measures identified in previous sections, this includes:
x Receptor resilience and recovery; x Event response; x Other forms of flooding; and x Upstream and downstream connectivity. Sequential adaptation to account for climate change also needs to be considered. Figure 10.1 attempts to capture some of these activities. The York 5-year Plan and slowing the flow initiative need to integrate with these wider interventions.
Figure 10.1: Flood risk management activities
56 These activities are frequently the responsibility of different departments within an individual organisation and, for other forms of flooding, even different organisations. Upstream management measures also require the cooperation of differing organisations and/or stakeholder groups. The key to efficiently and effectively managing flood risk in the SUNO and Foss catchments is to deliver the above in a coordinated manner.
10.2 Total Catchment Management
Additionally, there is a recognition that flood risk management cannot operate in isolation. It is linked with, and influences, a range of other activities and therefore needs to operate with consideration of those activities and work to achieve joint, multiple benefits. This recognition has spawned the Catchment Based Approach (CaBA) which is a community-led approach that engages people and groups from across society to help improve water environments. CaBA embeds collaborative working at a river catchment scale to deliver cross cutting improvements driving cost-effective practical delivery on the ground, resulting in multiple benefits including improvements to water quality, enhanced biodiversity, reduced flood risk, resilience to climate change and greater community engagement with their local river2.
Prepared under the Water Framework Directive, RBMPs were first published in 2009 and updated in 2015 with the aim of protecting and improving the water environment for the benefit of people and wildlife. The River Ouse is located in the Humber RBMP that identifies catchment partnerships which help to bridge the gap between strategic management planning at a river basin district level, and activity at a local water body scale. Catchment partnerships are groups of organisations with an interest in improving the environment in their local area and are led by a catchment host organisation. The host organisation for the SUNO catchment is the Yorkshire Dales Rivers Trust and the SUNO is part of the Dales to Vales River Network Area.
A number of main sector groups responsible for managing the water environment are defined in the RBMP including:
x Agricultural and rural land management; x Government agencies; x Industry, manufacturing and other business; x IDBs; x Local government; x Mining and quarrying; x Navigation; x Non-governmental organisations; and x Water industry. These have varying roles, including regulator, operator, influencer and undertaking projects. The activities involved are shown in Figure 10.2 below.
2 www.catchmentbasedapproach.org
57 Figure 10.2: Total Catchment Management
This places the Long Term Ouse (York) Flood Management Plan at the core of the approach, as it is perhaps the only catchment wide activity that has national and local government backing, and can draw the various partners and stakeholders together.
10.3 Recommendations
As a way forward it is recommended that:
x The Environment Agency consult with it’s partners and stakeholders, potentially through the RBMP catchment partnership to explore how a Total Catchment Approach can be formulated and implemented.
58 11. Summary
Over 600 properties were flooded in York during the Boxing Day 2015 flood event, which saw the second highest flow on the River Ouse and highest flow on the River Foss in the recent record. Since December 2015, the Environment Agency, City of York Council and partners have taken action to reduce the risk of flooding in York comprising the immediate recovery, developing a 5-year Plan to address flooding in the short term and undertaking this review of means by which to manage the risk of flooding in York in the long term, through strategic flood alleviation measures upstream of York.
Through the 5-year Plan, the Environment Agency has aspirations to deliver flood defences through the City of York by 2021. At the time of writing, Initial Assessments had been completed reviewing the standard of protection provided by the existing flood defences and assessing what measures might be required to improve the standard of those defences and provide defences for those properties at risk of flooding without any flood defences. Detailed appraisals of these flood defence proposals are scheduled for 2017-18 with detailed design and build of the flood alleviation measures through to 2021.
In parallel with the 5-year Plan the Environment Agency are also reviewing potential flood alleviation measures that could be implemented outside of the city. These more strategic, catchment-wide approaches to flood management are primarily focused on slowing the flow of the Rivers Ouse and Foss so that less water reaches York during a flood event and therefore water levels are lower than they would otherwise be. This document has undertaken a high level review of two key measures to slow the flow to York: upstream storage and NFM, in addition to acknowledging that other large-scale engineering measures exist.
There are numerous washlands along the River Ouse up-, and downstream of York providing storage of excess flows during flood events. There is an on-going study which is developing a new hydraulic model of the washlands downstream of York with a view to better understanding their operation and seeking to optimise their performance during flood events.
The review that has been undertaken here has concluded that there is merit in the Environment Agency undertaking a more detailed review of the washlands upstream of York on the River Ouse, to assess if the operation of the existing washlands could be adjusted so as to lower extreme water levels through York or if a new washland could be created to store additional water. The reach between Poppleton and the Swale-Ure confluence is the obvious location for such a washland, with the reach upstream of the River Nidd being preferable.
Work undertaken has identified that the flows of the three main tributaries of the Ouse: the Rivers Swale, Ure and Nidd all peak within a few hours of each other creating high flows through York. Separating the peak flows would therefore benefit York through lower water levels. The River Nidd is typically the first of the three tributaries to reach its peak flow and, on its own, is not capable of causing flooding through York. Therefore storing the flows of the Ure and Swale, and allowing the Nidd to pass through York whilst delaying the other tributaries in a new/improved washland, should result in reduced water levels through York in an extreme event.
A NFM study has also been undertaken focused on the River Foss (Appendix B). This study has found that there a many potential opportunities to implement NFM measures in the Upper Foss and Tang Hall Beck catchments however, there are comparatively few in the Osbaldwick Beck catchment. Modelling of some 130 interventions on the Upper Foss saw small reductions in peak flows and the timing of the hydrograph peak delayed. The major impact arose from the
59 implementation of measures to reduce runoff, noting that modelling of these measures has the greatest uncertainty associated with it. In contrast, implementing 54 measures in Tang Hall Beck resulted in comparable reductions in peak flow and crucially the runoff based measures did not dominate the results. Measures to increase roughness (e.g. large woody debris) contributed to approximately half of the reduction seen in the peak flow.
Ultimately, NFM appears to have a small role to play in flood risk management on the River Foss. However, it brings many other, wider benefits and therefore its application should be encouraged on the basis that it will be able to have an impact mitigating flooding further downstream.
Based on the modelling undertaken on the River Foss, and the literature review that has been undertaken (Appendix A), it is concluded that the sheer size of the Ouse catchment upstream of York will mean that NFM will not be able to influence the flood risk in York. That is not to say NFM is not valuable, it will make a difference in the immediate locality however any impact is unlikely to be seen in York, particularly the scale of interventions that would be required.
11.1 Recommendations
This review of slowing the flow measures to provide additional flood protection to York has identified the following recommendations:
x The Environment Agency to continue working with partner organisations to deliver the 5- year Plan to provide flood defences through the City of York; x Develop a broad-scale 1d routing model of the Swale, Ure, Nidd and Ouse upstream of York; x Use the model developed above to review the performance of the existing washlands upstream of York to evaluate how they can be used to support and enhance the ongoing work of the 5-year Plan by optimising washland operation, and the potential to create additional storage downstream of the Swale-Ure confluence; x Appraisal studies to be undertaken as part of the 5-Year Plan should consider providing upstream storage in the Foss catchment exploring locations identified within this report where structures over the watercourses constrict the flow downstream; x Partners including the Environment Agency, City of York Council and North Yorkshire County Council to work together to explore public acceptance of providing additional upstream storage; x A multi-agency NFM Implementation Strategy should be developed, integrating NFM ambitions with other objectives such as habitat creation and water quality improvements; x NFM opportunities within the Foss catchment should be supported, on the basis that they will provide a positive, impact in terms of flood risk; x NFM opportunities should be focused on the Tang Hall Beck and Upper Foss catchments; x NFM measures promoting runoff reduction, particularly farm scale land use management, and roughness based measures, in particular riparian planting (buffer strips or tree planting) and installation of Large Woody Debris should be encouraged;
60 x It is recommended that the Environment Agency appoint a liaison officer to work with landowners keen to invest in NFM solutions; and x The Environment Agency consult with it’s partners and stakeholders, potentially through the RBMP catchment partnership to explore how a Total Catchment Approach can be formulated and implemented.
61